Say you were a guardian or parent and get to decide when a child can get a phone or use a computer and get internet with it. If you wish you can also install software and change router settings to what you see fit.
Some parents decide to forbid the internet completely, others are more relaxed. Some go the helicopter route, and some do not care whatsoever what their kid does online.
What is your policy on letting a child use the internet?
Similar mentality here.
I’ve got some basic parental controls in place. They are intended at emergency buffers, rather than to stop a concerted effort however.
The best method is to teach and train. No security is going to be invincible, without being very problematic to work within. Children also learn fast, when motivated.