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.
Children will never "waste" their allowance on porn. It is just too expensive for most kids, especially if they can perfectly get what they want for free at the age when they are still exploring what they like.
Nobody that watches porn wants their accounts to be detected, doesn't matter if they pay with bitcoin or not. Bitcoin is not the bad guy here.