The internet is not a safe place and when a child watches obscene content that very well starts to warp their mind and perception. They hunt for further gratification and this can lead in to more murky content. Pornography can be malicious and internet pornography addiction is real too.
I am in favor of restricting content however for the reason that there really is nasty content which young children are discovering and it's leading them to grow up corrupt. If there was a way to divide the two but you can't, however the way it is it's either all or nothing.
Filter it, let a child hack around the filter expanding their computer skills and reward it. That it self is far more positive then being able to view all the content that is available. Innocence can be lost prematurely and that's disastrous for the child if it is lost.
Pornography nowadays isn't like it was during the MTV years which many will compare from.