My question is about the sleeping situation... how big are these homes? How many bedrooms do they have? How many brothers and how many sisters are there? It seems to me that typically, kids are divided up according to gender. Also - maybe with that many people in the house, they've all just learned to sleep through noises/commotion. There is bound to be someone getting up in the middle of the night to go potty, get a drink, etc.
The boy could have just been in a deep enough sleep to where he wouldn't wake up when picked up and if he did wake up, he was probably groggy and would maybe just think it was one of his parents carrying him?
Also - if there were problems in the house, children are pretty blunt and I would think they'd tell the truth to police officers about whether or not the parents ever acted aggressively... whether or not this little boy is a little daredevil and banged himself up a lot.
I also saw on a news report that they were dusting the back door for fingerprints (why not the front door? maybe the front door was dusted too, but there was something odd about the back door that grabbed their attention?). Maybe this is what lead them to Hopley?