That said, a better approach would be to limit kids under certain age from owning smartphones with full internet access. Instead, they could have a phone without internet access—dumb phones—or ones with curated/limited access.
Personally, I'm not too worried about what risqué stuff they'll see online especially so teenagers (they'll find that one way or other) but it's more about the distraction smartphones cause.
Thinking back to my teenage years I'm almost certain I would have been tempted to waste too much time online when it would have been better for me to be doing homework or playing sport.
It goes without saying that smartphones are designed to be addictive and we need to protect kids more from this addiction than from from bad online content. That's not to say they should have unfettered access to extreme content, they should not.
It seems to me that having access to only filtered IP addresses would be a better solution.
This ill-considerd gut reaction involving the whole community isn't a sensible decision if for no other reason than it allows sites like Google to sap up even more of a user's personal information.