If there was one thing Draco tried to pride himself on (Well ok there were many that the former death eater did, but for now..) It was having a romantic touch and knowing how just to make the love of his life's birthday even better then it would of been normally. No one (least of all Draco) would of expected that after school him and Harry freaking potter, the golden child himself would of hooked up but well, love was a funny thing.
Of course Draco letting Harry know that Ginny had been cheating on him with half of the boys in Slytherin helped free up Harry while forging a begrudging friendship that turned into Draco being there to comfort Harry and then well, one thing lead to anther.

Still, them living together wasn't always easy. Draco was used to doing things his way and Harry anther, but Harry was the bread winner while Draco could only get small temp jobs here and there due to his issues and naturally had been cut off by his family for dating Harry. Still Draco did what he could, cooking and cleaning around the house and doing the shopping though after spending a wee bit too much a few times and getting things they really didn't need, he was on a semi allowance of sorts.
It was the semi allowance and his restricted magic that had made getting Harry's birthday present ready in time so tricky. the ingredients were not only bloody expensive but they were also on a restricted list and he'd had to pull a few strings here and there. AKA: Beg Ron and Hermione for help when he explained WHAT he was doing, Though once he did they looked positively giddy to help. since they had never really gotten along with him, they were all polite enough when having dinner at each others house but he was never gonna go hang out with them on his own, he just took it as a sign they were coming around.

Finally it was Harry's birthday, and Draco had made all of Harry's favorites for dinner and gotten a bottle of his favorite wine ready. Harry didn't drink often but when he did he was picky about what he drank.
"I swear.. if he tries to whine it's not just treacle tart's again this year.." Draco mused to himself, holding a flask of the potion he had worked on for SO long and watching the door, worried Harry would be late. 
Of course he shouldn't of worried in the first place, you could set a clock to Harry's time table unless a case came up, in which case he ALWAYS sent a owl. As the front door opened and Harry came in, he gave Draco a huge smile even as he took a deep breath in.
"Somebodies been a busy I see~" he said, sliding his jacket and shoes off and slipping into a pair of slippers, setting his briefcase down and noting the flask in Draco's hand. "eh, what's that? not gonna have a few glasses with me because you got hungover last time?"
"N-No!" Draco huffed, cheeks burning and glaring. "It's a special love potion you git! I'm gonna make myself your ideal man for 24 hours..48 if I got the dose wrong." Draco huffed and blew a raspberry at Harry.
"DRACO WAIT!" Harry cried out, his eyes going wide but it was too late, Draco had already chugged it all. 
"What? your gonna tell me that I don't need to change for you? Harry this is a..a.." Draco felt funny all over and found himself falling onto his rump even as his clothes started to morph and change.,
His black trousers and dark green short sleeved dress shirt were apparently merging together while at the same time his boxers were both pull up closer to his crotch and puffing out.
"Y-yeah uh Draco.. my idea of a perfect man for me.." Harry started as the clothes merged together and changed color, turning a Gryffindor red onesie and his undies finished puffing out, giving a distinctive crinkle under said onesie, making Draco's eyes go WIDE as it clicked what he was wearing. 
"Is a big helpless diaper boy.. or adult baby." Harry finished, sounding both sheepish but a look of clear delight on his face.
If it wasn't for the pacifier, also red that popped into Draco's mouth right at that moment, he might of said something that gets babies mouths washed out. 
Instead, compelled by the potion he had brewed and drank, Draco suckled loudly and crossed his arms.

Harry was torn as he watched more attire changes, like Draco getting gold booties and mittens on his hands and then -squeal- a bonnet that was mainly red with a gold frill on it. He should be going and seeing if he could reverse the effects of the potion, since Draco clearly hadn't known what he was getting himself in for.. But he also just couldn't help but gush over how CUTE he was!
Even as he kneeled down and picked up Draco who apparently had somehow gotten lighter due to the potion (Harry's dream baby would be light enough to carry around with ease so of course..) a thought flashed into Harry's head.
"Wait a minute.. I know this potion.. Draco how did you get you hands on everything meant to make this?" Harry asked, looking Draco in the eyes.
He's of course notice he wasn't getting as munch back from the grocery money as he should of been and noticed Draco had been cutting back on personal treats lately but had figured it had just been for a present or two for Harry. and in a way, he hadn't been wrong.
"Draco, I asked you a-" Harry started, then noticed the look of 'what are you, daff?' on Draco's face, his mouth still working the paci. "heh, right. sorry." 
Tugging the paci free Draco worked his mouth around and stuck his tounge out over and over again, apparently trying to get the taste of the rubber nipple out of his mouth. 
"Excuse me, if ANYONE has some explaining to do right now, I thinks it's YOU!" Draco finally huffed, giving Harry a grouchy face. 
'I just had to love me a bratty little.' Harry mentally sighed then out loud. "If your not gonna answer my questions first, the paci can go back in while I explain." He said, still holding it. "we BOTH know you won't be able to resist it." 
"Nggghh.. Finnnne. I went and asked Hermione and Ron for help getting what I couldn't buy. They were delighted to help..." Draco sulked then his eyes went wide and he locked eyes with Harry, who was giving a pained sigh. "THEY BLOODY WELL KNEW THIS WAS GONNA HAPPEN DIDN'T THEY!?!"
"Yeahhhh I told them all about it in senior year. Don't worry Draco, I'm gonna give them a earful.. once the option wears off. There's no quick cure and I'm positive you'd rather me not have people over or go out with you right now.. you can't be left alone." Harry said.
"..Ok WHAT all am i in for here? Because it feels like I can't walk and I keep having to fight the urge to reach for the stupid pacifier! I think it's only because you want me able to talk I'm not bawling for it.." Draco huffed.
"W-Well my ideal little guy is a crawler who also likes being carried by daddy. he's totally helpless and dependant on me for everything while still being bratty at times. He's also..er.." Harry paused, trying to think of a tactful way to put this next part.
a loud and muffled poot came out of Draco's rear and the blonds eyes went wide, as LOTS more followed. 
"D-Don't tell me.." Draco mewed, closing his eyes tight.
"Yeahh.. my little guy makes me LOT'S of presents.. and by that I mean-"
"I KNOW WHAT Y-You.. you.. OH NOOO!" Draco snapped, squirming and whining as his guts were churning and trying to hold back, but the potion was stronger then his self control.
Harry held him out, hands under his armpit at arms length from him, the biggest grin ever on his face as Draco made the CUTEST 'I'm pooping' faces while the back of his diapers filled up and pushed out till.. yup, right on cue: the snaps of Draco's onesie gave out and it popped open as the poor blonds diaper filled up and sagged.
Draco whined and put his mitted hands to his face, both to cover it in shame and to try and block out some of the smell that clearly didn't bother Harry at all. 
Finally it was all out, and Draco was left panting and tired from the force of the mess he'd let out and the back of his diaper was sagging big time, making him worry about it breaking/sliding off and hitting the carpet. 

Harry just gushed and squealed like a fucking school girl and lifted Draco up higher so he could get a better look at the filled diaper.
"Oh wow Draco! you the best present maker ever! look at the BIG present you made JUST for daddy!" He coo'ed, clearly having to just enjoy having Draco as his little boy for the next day or two while Draco was feeling FAR less pleased.
All the money, time and effort he had gone though, just to end up a diaper filling baby? Hell, he could of saved himself LOTS of time and money and just picked up a pack of diapers and some ex-lax at the chemist's! 
"Who's my little diaper filler? who's my number one at making number too? who the best widdle present ever~" Harry coo'ed, bring Draco down and giving him a Eskimo kiss. 
"M-Me daddy!" Draco found himself giggling and smiling like a fucking moron, before he was lifted up. 
Thankfully there was one hail Mary in all of this: Harry liked a brat so..
"If you don't put me down and change me right this instant, I'm going to spit up on you."  Draco threatened. 
"..That's the perfect baby threat! AWESOME!" Harry cried out and pulled Draco in, the blond unable to keep himself from wrapping his legs around Harry's hips and hugging him, while Harry, used both his hands to smush Draco's bottom and squish the mess around... in a way Draco wasn't sure if liked or not.
'...what the hell.. it's only 48 hours..' Draco thought, giving in.

Elsewhere Hermione and Ron were having a drink and smirking. 
"So, by now Draco's likely fudged himself.. and Harry's loving it." Ron snickered. "As well as fuming at us for not warning him what he was getting into."
"heh, if he thinks he's mad about that.. wait until he finds out I substituted a ingredient here and there so this is permanent." Hermione said, smirking and her and Ron tossed their drinks together and laughed.

The end~