Which system should I use for a kid-friendly post-apocalyptic campaign?

My kids (10 & 13) and I are wrapping up our first campaign together in D&D 5e, and I'm starting to think about the next one. It's going to be a homebrew setting--future humanity decimated by climate change, but also elements of weird magic with giant plants and insects, inspired by things like Studio Ghibli, Kipo, etc.

After watching the recent Critical Role - Tears of the Kingdom oneshot, I started tinkering with my own system (PbtA based, with lots of opportunities for inventive crafting, and a video-game-inspired skill tree rather than strict classes) which is fun, but really time-consuming.

Wondering if anyone knows of an existing system that would work well for this setting? I'd like to find something simpler than 5e (which is the only system I know well), since they mainly enjoy the story and role-playing rather than lots of number crunching and detailed rules.