My five year old has small white bumps on his knees. Initially they look like enlarged pores but one or two may grow larger than the rest and remain for weeks. He has a large one on his left knee and several small ones on his right knee. I ve tried scraping them but he complains that they hurt. He s fallen and scraped them and they may come off or just the top layer comes off and a white layer remains underneath. Any idea what they are??