Well, you really won't find many decent games on steam for free, but I can still point you in the direction of a few. If you're into online multiplayer games, you might check out Robocraft. It was one of the first steam games I ever got, and while it's certainly not quite as good as it once was, it'll probably still hold your attention for awhile. You might also try creativerse, which is basically one of those Minecraft clones. I remember having some fun with that one when I was younger. Some smaller ones are Clicker Heros and You Have to Win The Game. I never really got super into either of those, but I'm pretty sure they're kid friendly. Oh, and I almost forgot about Brawlhalla, which is basically Super Smash Bros but with mythology thrown in.
Those should be mostly kid friendly, but I can't make any promises because I wasn't really playing them with that in mind, and I never really got around to finishing most of them in the first place. Either way, that should keep you busy for awhile.