I have also observed children sleeping. Below the edge of five they did indeed turn nose down from time to time, thus being able to turn right over and fall out of bed. It seems that what the child learns is not to turn nose down, probably because of the learned fear of suffocating. Young children also fall out of bed because they haven't learnt not to. They may do so quickly after going to bed as their first period of dreaming--and body movement that precedes it--occurs sooner after they fall asleep than in adults.