For a little boy playing with toys he looks depressed
This is what I was thinking and the pics from knox seem to me the boys look different. It is hard to tell but possibly thinner from what Knox said about depriving them of food/drink.JMO; I'm not a doctor, and don't even play one on the internet:
The pale skin and vomiting could have been a number of things. As a parent, I would've taken him to the ER for fear of meningitis. But as this pair didn't seem to even consider doing that, I fear it may have had a more sinister source, such as concussion.
The "thud" the older child heard just conjures all kinds of horrific images.
So one of the boys could have been sick from virus or malnutrition and then possible abuse.