If there’s one thing I learned both as a kid and as a father, it’s that restricting kids’ access to computers - or anything really - just doesn’t work: software solutions that exist for that purpose are almost always defeated by kids, who are reliably more clever than the adults who try to restrict them, and only exist to falsely reassure their parents.
If you’re serious about controlling your children’s cellphones, I’d suggest buying them Linux phones, or phones that you can install a mobile Linux distro on: nobody makes Linux apps, so good luck getting malware or shitty social media apps on them. And of course, you can keep the root password to yourself and set up your kids as non-privileged users.
Either that or feature phones - if you truly hate your children.