I think that's because the average JW isn't capable of building his own morality system.
Paradiseseeker made the point that first came to my mind.
There is also the risk of a child asking questions that the parent cannot answer. There is then the risk that the parent has to actually think about Watchtower rules, and come to see that they do not always make sense.
For example, a lot of JWs do not agree with the ruling against birthdays, but go along with it because they have no choice. I think the reason for this is partly because they are forced to explain the reason out loud to their kids, and hear just how hollow the reasoning is.
A child doesn't always accept "it is wrong because I/Jehovah says so" as an answer. Many Watchtower rules make sense some of the time but not all the time, because Watchtower is a black-and-white religion in a world of colour. What happens when an inquisitive child start asking "what if" questions. The parent is forced to confront the flaws in their doctrine.
Think of the situation where a teenager asks about abortion. "It is murder, it is never acceptable." But "what if it is girl at school, and she can't care for the child? What if they were raped, and can't bear the idea of seeing the child? What if the girl is likely to die giving birth?"
Watchtower doesn't want parents thinking. Just give your child an ipad, get the child to watch the same piece of propaganda hundreds of times until they are too indoctrinated to ask tricky questions.