Holy shit, guys. Don't laugh but my kids and I just spent two hours playing Tank! Tank! Tank!
It's the dumbest fucking thing ever but TOOOOONS of fun. It's great, too, that it is F2P but it limits you in maps and modes and to only 3 games per 12 hour period. Seriously, though, I broke my rule and bought some DLC to remove the time limit and get new maps.
It really is lots of fun even if the graphics look last-gen. Really, it plays very much like Earth Defense Force. Big, dumb enemies in huge levels that look like 90's era 3d gaming Lego playsets and everything blows up really big and crappy.
Only bought the one map so far and I'm not sure if I'm gonna buy it piece-meal or just buy the original boxed copy off of Amazon. It really is terrific local MP for kids.
I tried to get them to play NSMBU and, even though they can hold their own in the Left 4 Dead games, the platforming was just not clicking with them. They were just kinda fucking around and dying a bunch so I remembered I heard that TTT went F2P and thought I would try it out with them.
Each player gets to take their picture and put silly costumes on it, and I don't need to tell you that kids love that kind of shit. Really, I know the game has a crappy reputation but I recommend you give it a go if you find a need for dumb, fun local MP.