Sites should empower parents to control that. Have a sign in by default and an option to ask the site to block ips associated with you to be able to sign up for more accounts.
Are there ways around it? Sure. It’s a just a lock to keep honest kids out. If your kid doesn’t feel comfortable about asking about stuff like this or feel like they have to around you, you aren’t going to win, they’ll find it, and it’ll be a blast for them. If you talk about it, at least acknowledge the issues with it, say when it is and isn’t appropriate, etc it’ll do leagues better then all bans and censorship attempts in the world.