No crime is being commited when a child/parent uses the others FF number. Show me a law that states this. The worst case is you will not get the miles posted to your account. There is no way to prove that you did not accidently mix up the two numbers.

How many children out there have the exact same name as their parent? Not that many for the airlines to be worried about.

As long as the people involved are aware that it is not allowed per policy, they can make up their own minds as to what is the right thing to do.