I assume the person behind the camera is an adult who is not completely without a brain. These kids were not alone when they fell asleep. I have myself spent a minute or two taking a picture of a funny situation before helping the kid to bed, without experiencing any fatalities.

(Also: A kid that tired is not likely to get much stuff past his/her lips. Suffocation usually occurs when you try to swallow something, and it gets stuck in your throat. These kids are not sufficiently awake to try swallowing!)