As a few before me mentioned, the first game that inevitably comes to mind is Super Smash Bros. Ultimate
. It's very accessible, it features characters that they will know and love, and it's fast 'n flashy in all the right ways without ever looking or feeling violent.
is basically a free-to-play indie adaptation of that exact style of fighter. The roster isn't as recognisable and in my opinion it is never going to look or play as good as Nintendo's original, but if you don't have access to a Nintendo Switch it can't hurt to give this one a try. It's free, after all.
Personally, I think I might give the nod to Lethal League
and Lethal League Blaze
(sequel). It's a fighting game of sorts where you and your opponent(s) hit an anti-gravity ball at each other until one of you gets hit. There's no limit to the speed the ball can pick up with each hit, so the action gets more frenetic constantly. It's really good fun!