Your kids & playing (or not) sports w/ head impact

Just curious if you guys allow your kids to play collision sports like football, boxing, MMA, etc. with the information now available on head trauma and its ramifications for some. And if you let them play, do you ever worry about possible damage to their brains or not?

I've heard some popular ex-NFL players (Troy Aikman & Terry Bradshaw come to mind) say they will not let their kids play football now that they know what it does to (some) people's brains. Other players say they have no problem with it.

Have any of you thought of this and/or changed your mind with the new info on the impact of head impact in sports? Or do you feel it is overblown and let your kids play whatever they want?