I read a study one time (I'm sorry, I cannot for the life of me remember where) that stated tests showed that kids who bang/ed their heads tend to be more intelligent than those who don't/didn't. I don't know how accurate this is, but hopefully, it'll make you feel better.
Also, I did a bit of research on this and most places say that if it gets really bad, slap a helmet on them. I laughed when I read that - I could just picture myself setting my 2 year old up with a helmet and a walker in a padded room. LOL