Adopt a Bat Dad
AKA "Danny becomes de-aged in Gotham and finds the only person he knows who can probably help. Bruce Wayne, the Batman. Except Bruce thinks Danny is a kid mistaking him for his dad??" prompt idea!!
HC that Bruce Wayne and Jack Fenton look super similar. Therefore, Danny and Bruce also look pretty similar!!
I love the idea that Danny already knows Bruce Wayne is Batman. Maybe it's his aura or because the amount of kids Bruce has directly correlates to the amount of bat-themed sidekicks there are. Who knows? Anyway, Danny comes into a small bit of trouble. He may or may not have insulted an immortal witch who cursed him because he's an "immature child, may as well look as young as you act!"
So. Now Danny looks a solid 3-4 years old. It's a good thing that Sam and Tucker briefed him on all he celebrity gossip before he came to Gotham, because he coincidentally knows where the Wayne Enterprise building is. He... can figure it out. Probably. It's actually alarming how many people watch what they think is an unaccompanied kid huff and puff his way in downtown Gotham. (Also, wow, Danny severely underestimated how difficult it is to run after being babified.) But he does make it to the general area of where WE is supposed to be!
His legs are practically shaking at this point, sweating through his toddler-sized NASA hoodie, and searching frantically for Bruce Wayne. Because he really didn't think of it before, but it's Friday afternoon. What if Mr. Batman isn't at WE today? What if Danny gets to WE after 5pm and he's gone until Monday? Would Danny even be able to find the Wayne Manor, much less get transportation there?
Except as Danny's becoming increasingly worried (don't cry, don't cry, don't cry), he spots... his dad?? in the coffee shop windows beside him. No, not his dad. Bruce fucking Wayne! Hell, yeah! Danny smacks open the doors of the coffee shop with single-minded toddler-clumsy determination. Makes a bee-line straight to the coffee pick-up. Bruce Wayne is standing off to the side, quietly speaking on his phone, as Danny practically slams face-first into his knees. Thankfully, it doesn't take either of them down, but it is particularly embarrassing.
Especially when Danny clutches to Batman's pant leg and confidently shouts, "Batman!" Except... he doesn't. A weird jumble of words come out of his mouth that sound more like baba! It's like the world screeches to a stop because, first of all, what the fuck. Second, that bitch witch! She must've made it so whatever he says comes out in toddler-speak despite the fact that he should be able to say somewhat comprehensible sentences, being he's physically 3-4.
That doesn't stop him from trying, though, so he ends up babbling baba, baba, baba in an increasingly frustrated tone.
And Bruce Wayne, who's become used to Damian calling him baba instead of Father, can only stare down at this child who could pass as his clone. The similarities are striking. Even if the toddler is huffing, red-cheeked and clearly on the verge of crying, he looks so much like Bruce that he wonders momentarily if it's another Damian situation.
Regardless, there's a kid crying in front of him, tugging on his pant leg and calling for his dad. And Bruce is nothing if not absolutely weak-hearted against stuff like this. So, he leans down and just... scoops the kid up. Murmurs, "Shh, it's okay, kiddo. You're okay." Pats the kid's back, sways. Completely forgets he's in a crowded coffee (this is definitely going on YouTube, posted under 'Wayne Adopts Another??') and that he's on a phone call with Dick. It's like his Dad Instincts kick in and he's completely focused on Danny.
Danny is... bewildered. Because why is the Batman coddling him?? Except he notices that others have noticed, and have their phones out recording, which is really Not Good. He's not super confident that his parents would be able to recognize him while he's de-aged, but the fact that they might? That's opening a can of worms he can't handle at the moment. So his little string bean arms loop around Bruce's neck and he shoves his face into the collar of the man's suit. Much to his irritation, he can hear several girls next to him coo and giggle about him being such a cute baby. Danny's really regretting not approaching Batman privately now.
Bruce calms the kid down and then immediately goes to the store manager, asking if any parents have lost their child. He doesn't trust that someone may claim Danny as theirs when that may not be the case. Then, he calls up Gordon, asks about any missing person reports on a child the ages of 2-5 with average height, medium build, and black hair. No hits. Eventually, Bruce makes up his mind and takes Danny home with him. Oracle will likely be able to pull more information than the GCPD anyways.
Meanwhile, Danny zonks out. Like full on, toddler-sprawl open-mouth drooling, because it's been a long day and he got Batman. He did it! And from the way Bruce is still carrying him, Danny will likely be with him for a little while. A little catnap will do him some good. Maybe when he wakes up, he'll magically have the ability to speak normally instead of hysterically babble.
(Four hours later, Danny wakes up on the couch at Wayne Manor, bundled up in super soft blankets with Bruce talking on the phone with some woman. Bruce smiles at the way Danny says baba again. Danny's ready to throw that witch into Bruce's well-maintained fireplace because screaming son of bitch isn't as satisfying when it sounds like sa-bA-BAH!!")
Cue Danny doing increasingly ridiculous things to make The "Greatest Detective" Batman realize he's not a literal baby and Bruce Wayne so enamored with this little kid that he does not realize.