Kids don't have to be that big or strong to get an injury from their heads colliding into one another, though.
Sure, they can still get hurt, but the skull protects the brain. The normal kid in pee wee football isnt hitting with enough force to continually cause head injuries. Again, I would like to see examples of adults with issues from only playing pee wee football.
Soccer and baseball cause just as much, if not, more injuries in youth games. Its one reason why the girls had to ban heading until a certain age.