Try not to worry although I know it's easier said than done - by now they are strong enough to move themselves into a position where they can breath and I know both of mine liked to sleep on all fours with their bum in the air as soon as they could!!
My neice still sleeps bum in the air and she's 4! It's sooooo cute.
Ellis has just started to sleep on his side or front. I kept rolling him back but he's just go right back again so I've given up. Sometimes he has his face down into the matress which scares the pants off me but if I move him, he just puts himslef back to where he wants to be.
My DS has also slept on his tum since he learnt how to roll over, at about 6 months or so. I was really worried to and initially spent my nights turning him over onto his back again, which just woke him up out of frustration. Tears all round, and not just from the baby. Now I just make sure he goes down on his back and leave him to it. He also does the bum in the air pose, which is just the cutest thing. I also sleep on my front so I don't know if he gets it from me, although I don't stick my ass in the air. Don't think it would look as cute as when DS does it!