Given that your other kids are both full casters, you can go ham on a melee class without fear of overshadowing them. That plus KISS screams Berserker1 Barbarian to me. All the advantages others have described of barbarians generally ("you get to be angry and yell, and it means you hit harder!"), with no fiddly (for a five year old) once per turn damage boost like the Zealot. If not, I'd second either Champion Fighter, or maybe Assassin Rogue if you're generous about granting surprise.

(1) Just drop the exhaustion. Simpler, more fun, and not going to break anything in a game for kids.