Simple fact: kids are going to look at porn, deal with it. The more you lock them down the more theyre going to rebel.
My dad tried to enable parental controls on AOL when I was ten. I then installed a keylogger, got his password and disabled the parental controls. l hid porn on our main computer in plain sight for years and no one knew (this was 10-15 years ago). Giving a kid a smartphone and telling him not to look at dirty things is essentially giving them the motivation to. Its all reverse psychology.