+1 for parent controls
Apple parent controls
I can understand that you might want to know if they are asleep or not before you enter; you wouldn't to wake them up just as they are drifting off, but being your own child there won't be any privacy issues. As suggested, you could just as easily have a digital video monitor, so using the mac is no different except the idea that you could be doing it secretly and the dedicated monitor is obvious.
Of course, did you ever do anything like this as a child? Did your parents do anything to provide a video feed from your bedroom? How have you turned out [rhetorical]? There are times they will be a cheeky monkey, but is a video really required. It's great as a child to sometimes do these things and as long as they generally behave themselves, probably not worth having a video feed. Torch under the bed with a comic? It's just a modern day equivalent.
It is indeed possible to get a snapshot from the camera, users logged in or not, but I'd say think before you really choose to use this. Certain companies have used such things for stolen laptops, but probably touching on a tricky area even under those circumstances. Might retrieve the stolen goods, but probably end up with a reverse lawsuit against privacy.
imagesnap
Could be used in an office though, when the office is shut, for CCTV style purposes, perhaps, or your home machine when you are on holiday.
