I don't know if it'd be either of these since they're not a perfect match but these two both have the kid thing:
What Happens When the Second Male Lead Goes on Strike - not BL so he isn't ML but is the OG ML, don't remember if MC avoids him or not
https://www.mangago.zone/read-manga/what_happens_when_the_second_male_lead_goes_on_strike/
Full Moon Alchemist - I don't think MC knew his identity as an adult so he doesn't avoid his adult form as far as I remember
https://www.mangago.zone/read-manga/the_moon_painting_alchemist/
both are pretty good either way though
He was an actual child (at least I think, I'm still letting this marinate) but, maybe this? https://www.mangago.zone/read-manga/regas/
Yesss thankyouuu
I don't remember much but I think it was a bl? And the seme had a curse or something where he looked like a child and the mc saved him while he looked like a kid but then when the seme turned back into an adult the uke was terrified of him bc he was a known tyrant and the uke had been avoidin him or something idk sorry I think their were also a few side characters who were like powerful heroes or something