A lot of people focus on "kid friendly mechanics" being extremely simple or minimal, but the truth is those are just "good mechanics." If anything, I found that kids have a much higher tolerance for mechanical complexity on a regular basis in their games than I do, whether it be in videogames or tabletop. Your pair are a little young, but I can definitely expect them to be looking for more moving parts to play with before too many years go by.

If you're looking into maybe branching out into sci-fi, T HW does one of my favorite sets of rules for playing at all kinds of scales in their 5150 line. Whether you want to do ground-based stuff, Marines storming spaceships looking for pirates, fighters, capital ships, giant mecha brawls – they have something in the line that can take care of it and the mechanics are pretty straightforward and simple.

I was always a bigger fan of science fiction than fantasy and my library kind of reflects that.