This is a stopgap at best, and to be blunt, it's naive. They can go on their friends' phones, or go to a shop and buy a cheap smartphone to circumvent the parental controls. If the internet is locked down, they'll use one of many "free" VPN services, or just go to school / library / a friend's place for unrestricted network access.
Parents can only do so much, realistically. The other parties that need to be involved are the social media companies, ISPs, and most importantly the children themselves. You can't stop them, but they need to be educated. And even if they're educated and know all about the dangers of the internet, they may still seek it out because it's exciting / arousing / etc.
I wish I knew less about this.