@timorl I've had a lot of fun playing fighting games just by mashing buttons at an even older age so I can see the appeal for young kids. There's also a very high skill ceiling if someone decides to get serious, Skullgirls is pretty well designed from what I remember (plus there are a lot of similar games to choose from, like Guilty Gear, BlazBlue and Under Night: in-Birth series).
If you're looking for other colorful, split-screen multiplayer, easy to play (or not? I haven't been 4-6 y.o. in a long time xD) games, there's also Runbow (I remember that one from Wii U times, but I see it's available on steam as well).