@PTR_K Well, of course every group and individual has their own rules preferences. I kinda like GURPS, it fits my mental model quite well.
But my conception of “stonepunk” wouldn’t mean ice age/neolithic in a pure/historical form, as I’m not that fond of Quest for Fire/Clan of the Cave Bear storylines. Mixing in a bit of spirit-based religion or even Sword & Sorcery would be possible, but my conception of “stonepunk” would go quite beyond that. Meso-america mixed with early dynastic egypt and mesopotamia, with some anachronistic tech based on “weird rocks”.
@Hollow_knight I did my last P-A game with GURPS, which unless radically stripped down ain’t exactly kid-friendly. What’s your core story in this game? Crafting and skill trees sounds almost as complex as 5E, especially given that the kids already know the latter… Might have a look at some of the simpler derivates of 5E (Five Torches Deep, Shadowdark), create a few custom classes…
I’ve never been a big fan of PbtA, so I’m not a big help there. Within that rules genre, I’d have a look at FATE Condensed plus the cliffhanger mechanic from the Masters of Umdaar setting (PDF available for free).