King of Tokyo. My two boys have a screaming good time playing this one, and it doesn't really matter if they win or lose.

I'm also having a lot of fun playing Battlelore with my oldest. It's just on the edge of comprehension for him right now, but he's rapidly getting better. He beat me by a good card draw last week, and he felt so good doing it.

Really, anything that has activity and allows for good choices is fine. Their strategy sucks at that age (really, any age up until about 8-10), but just the act of doing something fun with the "big people" has bonuses.