Hi there, just wanted to add - both of mine slept on their tummies as soon as they could roll over. It helped DD cut out the light so she actually slept better once she started to roll over. DS will also sleep on his front, though we have all sorts of fun and games with him.
In short, I wouldn't worry and just give them lots of time on their fronts during the day then they can learn to roll front to back and push themselves up.
X