I think those two things are related because you can buy porn with bitcoin account without being detected, but this is one way the use bitcoin is not good for children 
Well then, parental guidance is needed. Children should not know that bitcoin can be used for porn and most of all they should not know that they will not be detected if they use bitcoin as a currency for paying porn. Or their mothers will get really mad.
If the children are smart enough to pay for their accounts on porn sites then they are smart enough to see that Bitcoins can be used as a medium of payment. Also various sites display all available options through which payments can be made.
Also Bitcoin is just similar to any other type of currency in the sense that it is just as hard to earn as a fiat currency. It's not as if any child can buy himself an account with Bitcoin lying around him.