Is there such a thing as too clean when it comes to your kids?

I have a "friend" who is constantly following his kid with a bottle of hand sanitizer. The kid does not know how to get dirty. He's 4 and very skiddish when it come to anything on his hands. My daughter is a little younger and I invite her to play in sand and get messy. I think all of that is a great part of growing up. What do you think- is there such a thing as being too clean???