They also have to fall on their face, so they will know not to do it again. Shedding a little blood is a good thing as long as it doesn't do permanent damage.
Also, they have to fall on their face so that they know that it's not that big of a deal. I feel like overprotective parents who treat every minor accident like a major catastrophe are doing their children no favors. If kids get the impression that each minor setback is a terrible disaster, they'll grow up with fragile mentality. Kids need to learn that falling down (figuratively and literally) is part of life and that they just need to get back up.
(as someone who has no kids and probably never will, I have no idea what the hell I am even talking about.)
-k