i've been sitting on this one for a while because i have so many thoughts on their childhood but even in aus i always try to write it as they both have a bit of a traumatic childhood before they got "adopted" or found each other so i'll include hcs from before and after that

(and this will be au only since canon childhood thoughts are. a different thing entirely)

ivan:
- lived on the streets for a while and had to learn to survive really quickly. ate some rats. both of his hands are permanently fucked up from that time he tried to steal and got his hands stomped on so he can't do many things that require fine dexterity like till
- lives a really minimalist lifestyle now thanks to his childhood; if he does have anything important, he usually keeps it on him or at till's
- when he got into an orphanage no one picked him until his first and last Rich Parent did and they just largely left him alone because he was a pick for optics (but he does well under instruction so slowly that parent started investing more in him but still off on the sidelines)
- can't cook because he has and will eat literally anything
- got really sick often as a kid
- no sense of object permanence until till came around

till:
- was shuffled around A LOT of foster homes and orphanages because he kept running away/people didn't like him
- always very active as a child and also loud
- favorite place to go was a guitar shop nearby just to look at them and the music sheets and stuff. after years of this the owner taught him a few things and the rest he picked up on his own
- can cook because he had to find some way to make food palatable for him
- other than food he wasn't very fussy at all... slept well ate well moved around a lot
- met ivan at an orphanage (till came later) and they only started talking to each other/hanging out with each other because everyone else was too loud
- just two boys sitting in a corner together scribbling on a notebook how cute
- never learned The Skills Of The Streets like ivan because he was always in a shelter and didn't have much of a need for it but he DID get robbed quite a bit so he's very protective of his belongings and knows where everything is at all times

i think that's enough for now... thank you for the ask sorry it took me so long to answer!!