Dirt Bike FAQ

My six year old has never ridden anything other than scooters and his bike (no training wheels) and a skateboard. I want to buy a four-wheeler (quad) for him, but have heard that a dirt bike might be safer for him, for the fact that if something happens, he can jump off. But with a quad, he would have a harder time getting off of it.
I am a little afraid, but want to start riding myself, and though it might be a good bonding hobby for us to share.