My son has always been a good sleeper but he does that in his sleep, too. I've seen him sit up, slump over and ooze back down on his tummy but by the time I get in to check on him, he's already totally out again. Is your daughter still sort of sitting up when she falls asleep again? Does she not lie back down? If that's the case, I would think that she couldn't be flat on the mattress so she should be able to breathe well. Even if she is lying face down but knows how to roll and sit up no problem, I would think that she would be fine.