Some kids just really need to sleep on their tummies. My younger child could not settle and had a really hard time. I talked to the doctor about it and he told me to try putting him on his stomach to see if it helped. Wow - what a difference! I talked to the doctor about the new development and asked about SIDS. He told me that it's best to put newborns on their sides or backs, but if they simply cannot settle that way, it wasn't the worst thing in the world to switch them to their stomachs as long as we were judicious about not having the room too hot, things in the crib, etc. My son, as a teen, is still a stomach sleeper.
Dawn