Simpsons is too hard for small kids imo. Content is fine, although the drain calls can be a little brutal (It’s over? Already??), and it’s almost certainly the deepest game here. So deep you’ll probably never see it all.
South Park is much easier, but it could be kinda crass for kidlets... although it’s from so early in the show so even if your kids watch SP now they might not quite get it.
Family Guy is a weird pin, but it’s a treasure trove for fans of the show, there are so many calls. It’s still probably a little tough for the kids with it’s short, very asymmetrical PF. It has a lot of messed up content too.
X-men is probably the safest bet content wise but man the magnets make this game kind of a bastard. Still, it’s a straightforward layout and has some nice ramp-age, plus kids love superheroes, although these are much more the OG 90s X-men vs any modern take on the team.
Sounds weird but putting content concerns aside, I bet your kids would have the most fun with the easy play of South Park, although you’d probably prefer TSPP or FG... and X-Men is probably the happy medium. Tough, but straightforward.