If you have your childrens iPhones as part of a family setup you can also block their access that way.
However I salute you for your choice, which is the harder and better way. If more parents did as you, we would not have the issues we have today.
Porn is nice enough but watch, but because there is essentially no discussion about how to have sex (outside of school, which at best is put tab a in slot b, here is how not to become pregnant, this is what STDs are.) the only input too many teens get is porn.