I think parenting is the key factor. Bad parenting will result in the child having problems, whether or not there is porn around. Good parenting will teach the child how to conduct their lives, even if they run across -- even if they have an premature interest in -- porn.
Fair enough about the parenting. So do you feel that all pornography is OK or do you think that some pornography crosses the line of obscenity and should be restricted? (I'll assume that you are opposed to child pornography and will accept that's a different thing because children can't give consent.)