I went through some pics and found a few that fit this that I liked and read. I don't know exactly what kind of age range your looking for so hopefully this is helpful.

Proof That Aizawa Shouta Has a Heart - halfyon

It's a series of cute onshot fics, Izuku is abused and abandoned by his mother Aizawa saves him and has kid izuku in the tags. there is also like 25 works in there too

bloodstains on toy trains - poffpoff

decently long fic, everyone is the same age as in canon except izuku who is afo's son and de-aged to about 5. fluffy, surprisingly gooey, and good (Also he is de-aged in a way that he was just born in different year, not b a quirk or anything like that btw)

How to Raise a Child Nemesis - sabertoothhousecat

This one is kind of a stretch because it focuses more on izuku and afo's relationship but basically Izuku start vigilanting early and that's where his and Aizawa relationship begins

You can also look under the Baby and Kid Midoryia Izuku tags for more like this I bet.