Adding to my previous posts on “Deaged Katsuki Would Be A Demon Child”. What would happen if Katsuki wasn’t the only one deaged?
Let’s say Katsuki gets deaged with someone else during a mission, back to around 6-7 years old. It’s only him and the other person, and they’re in a hideout swarming with bad guys. The villains go after them thinking they'll be easy pickings now.
If he’s deaged with Izuku, then Izuku has no idea he has a quirk, and goes back to his childhood fear/being frozen in the face of dangerous. Alone, he would’ve probably been easy to kill. However, he is not alone. He’s with Katsuki.
Unlike Izuku, Katsuki is a hell child with explosions coming from his palms, too much combat knowledge for a 7-year old, no self-control, and A Willingness To Kill. 7-year-old Katsuki has no concept of mercy, he’s determined to destroy all the bad guys and get his crybaby friend to safety, and the villains quickly realize They Have Made A Mistake. At least older Katsuki was obligated by law to only do so much damage.
Kid Katsuki protects Izuku fiercely, getting both of them out of the villain’s base and to safety. Maybe he even gets hurt in the process, but he's okay because heroes have to get hurt sometimes to win the fight! Everyone is torn between being impressed and scared, because holy shit, maybe a 7 year old shouldn’t have constant unlimited access to explosives.
The deaging quirk will wear off on it’s own eventually, so in the meantime Katsuki is bandaged up. Both of them are told they are students at UA, and All-Might lets both of them know that Izuku does actually get a quirk, but it’s a very powerful and somewhat dangerous one which is why it took so long to come in. Excited by the knowledge that they are actually becoming heroes together, Katsuki and Izuku spent the rest of their time as little kids running around together causing chaos. Class 1-A watches as they play kid games, go on “missions” together, and nobody misses how little Izuku looks at Katsuki with stars in his eyes. Just like big Izuku does.
After having seen Katsuki take on multiple full-grown adults and win, the rest of the class is starting to understand why.
Or
Another option is he gets deaged with Shouto. Katsuki doesn't know who this weirdo kid is, but he still does everything he can to keep Shouto safe. That's a hero’s job after all!
Shouto is torn between letting this loud but nice explosion kid protect him and protecting him back because Shouto is supposed to be a hero too! Ideally the LOV isn't the one behind this attack, but they are nearby. Or, at least one of them is.
The fight is turning nasty when Shouto hears Dabi’s voice and he recognizes his big brother’s voice, putting Dabi in a really fucking awkward spot. Dabi’s not in Shouto’s line of sight, but he can see Shouto and Katsuki, and he can absolutely hear Shouto calling out for Touya. At first he tries to ignore it, until little Shouto demands his big brother stop playing around because it’s not funny anymore, Touya. Where are you? Please, it’s really scary here! Big, round, mismatched eyes start welling up with tears, and for all his heartlessness, Dabi is only so strong.
Dabi ends up rescuing both of them, burning the people they’d been fighting to ashes. He gets them both to safety, and it’s only then that his little brother is able to get a proper look at him. Obviously, he expects that to cause some issues. Kid Katsuki isn’t so sure of this strange man, but Kid Shouto asks one quiet question as he stares into familiar blue eyes.
“Is it- Is this really you Touya-nii?”
“Yeah. It’s me kiddo. Bit hard to explain right now, but I’m still me. I promise.”
After several long seconds of silence, Dabi nearly ends up in tears himself when his little brother reaches up to touch his scars with a gentle little fingers. Little eyes watch in wonder as he confirms they’re real, and then Shouto gets super excited because he thinks Dabi’s scars look super cool. It’s so awesome that he’s not the only one with a scar anymore, and his big brother has gotten so big and strong! With how he took out all the bad guys back there, he must be a hero by now! Katsuki agrees, he thinks Dabi’s scars look badass and he’s a bit jealous that Shouto has a hero for an older brother.
Giving in to temptation, Dabi runs around with the two little kids for a day or two, playing keep away from the very worried heroes. They have an awesome time, and Dabi decides he likes the explosion kid too. A small part of him wishes it could stay like this forever, but it can’t.
When it’s almost time for the quirk to wear off, both kids are exhausted. Once he’s sure they’ll be safe, Dabi drops two sleeping troublemakers off right outside UA, where they’re found mere minutes before they turn back into teenagers.
Nobody knew Dabi had them the entire time, and neither Katsuki or Shouto recall what happened when they were deaged. The only hint they have is that kid Shouto mumbled out a very sleepy “Touya? You’re back again?” when Kirishima finds them. In the end, everyone assumes Shouto and Katsuki must’ve found their own way back to UA, and that’s that.
Dabi knows what really happened though. He’s got a few new photos carefully hidden on his phone to prove it.