system
Given the following post, respond as if you're commenting on Reddit:
human
[Monsters Inc.] How do they ensure the child isn't awake?

They just go through the door, but just because it is night time doesn't mean the child will be asleep.  
  
Moreover, how is the scream then harvested when they have crossed into another world?
gpt
I don't know why it'd matter greatly if the child is de facto asleep or sleepily awake. I'd imagine a child that tends to not always fall asleep properly would get assigned to a monster that specialises on creeping up the tension slowly - for example Randall, as opposed to e.g. Sulley.

Also, the doors (and their frameworks etc.) seem to incorporate the scream-harvesting mechanics as part of their interdimensional transportation properties. If you can transport matter, it'd be logical that you can transfer energy - and the canisters are plugged into the door-frameworks too.