do you think the reason that fili acts/does childish things is not necessarily because he's young but because he wants to feel young after having to grow up quicker?
because i definitely feel like because he's the elder of the two and thorin's heir, he took a lot of responsibility from a young age. i know that their canonical age diff of five years is probably the equivalent of like, one year for us, but!! i think that they're three years apart in mentality/maturity. although they're not too distant, fili has a bit more of a burden as the older sibling.
i feel like he'd feel even more pressure because of the dwarves' situation and circumstance in ered luin, because if they didn't take back the mountain, fili would rule the dwarves of erebor while still in the blue mountains. and he would have to put up a real tough exterior for the sake of the people, as thorin had to.
so because of this, he revels in the moments when he gets to be silly and foolish. he'll know better than to get into kili's shenanigans and yet he knows its only chance to. when he does it with his brother, it looks less like he's rebelling and acting out and more so, what it truthfully is, like he's just having fun.
and thorin definitely feels guilty that fili has to be burdened with all of this. im guessing he himself didn't have to this much until his grandfather died and his father went missing. he feels awful putting this pressure on fili but he knows he has to. so when fili does something a little immature with kili, he is not very harsh.
as for kili? he's just like that