I play poker with my son (he's 9 now) all the time. We've been playing for at least a couple years. The majority of the time, we just play for chips, but he's played for real money multiple times. A few times, I have bankrolled him. That's usually when we have a family game and the buy-in is like $20 (which is a bit beyond his reach). But, usually, I make him use his own money. We play low enough, max buy-ins of $2-$5 that he doesn't get too hurt.
He doesn't like losing, which is part of the reason I think it's good for him. He needs to learn to lose gracefully. We talk about the math, the strategy, discipline and other things you need for the game. We use logic and rational thinking. When we're playing heads-up, he says it's more like being in school than playing a game.
I think it's been good for him.