I would guess outside dirt too. We spend alot of time outside and I request play clothes if at all possible. I also recommend dark colored socks. They spend alot of time crawling around on hands and knees even when they're 4 and 5 but especially at the younger ages.
Most of my kids have 3-5 daycare outfits that are easy on and off and parents don't care about. They have found that works well and saves the nice things for the rest of the time.
