...so ban kids from having smartphones
Obviously it's not a complete solution, but you remove a major part of the problem by stopping kids having smartphones. They can have non-smart phones if actually necessary, noting that yes they do tend to have cameras now but the important bit is the reduced convenience and unavailability of the internet.
If you want to be nanny-statishly cynical then added pressure to stop people ignoring the ban is easily added by lots of public 'endangering your child' statements etc though if it comes from government people might ignore that by default.
I might change my mind if I ever saw any justification for a child to have a fully-functional internet-accessible two-way audio/video communication device which has only the barest of useless protections from others as well as themselves.