@saisai-chan I’m not sure what kind of platonic fluff you were asking for but here is a bakudeku one as children:
Their mothers have taken them to a park because they wanted their boys to have fun together outside since it was such a nice day.
Their mothers were watching over them as Izuku and Katsuki played together with their matching All Might action figures. They would run together while holding out their toys in the air occasionally shouting out “For I am here!!” or “Plus Ultra!!”.
Both wore bright smiles while yelling because that is what a great hero does, just like All Might. Recreating the battles he fought and saving the day. They did so much together before being called for lunch.
Katsuki would animatedly describe to their mothers what they did and the battles of the number one super hero, with Izuku filling in details here and there that Katsuki would miss in his excitement. Their expressions were bright and something to treasure.
Once done they continued playing as before until they came across a tree that was short and sturdy. Luckily for them that meant it lowest branches were within their reach with just a small jump. Katsuki climbed up first and caught their figures as Izuku threw it up to him. Izuku soon followed after him and so they climbed the highest they could.
They talked for a bit about their journey getting there and, of course, of All Might. That topic soon led to their desire to become heroes themselves in the future.
“We will be the best heroes, right? ”
“I know I’ll be the best.”
“What about me?”
“…You’ll be the best after me.”
“We’ll still be together, right?”
“I won’t need help beating the bad guys!”
“I know you’re going to be strong, but I’ll be strong too. What I want to know is if we’ll still be together…as friends.”
Katsuki looked at Izuku then his action figure then the sky. He wasn’t exactly sure what to say. He just knows one way to answer that.
“Yea…We’ll be together as friends in the future.”
“Yea. I can’t leave a deku like you alone! So…so don’t you go anywhere either!”
They are cute children but they are also impulsive. They were stuck in the tree. They honestly weren’t sure how to get down so their moms had to get them down instead.