Those are the ulterior motives people I referred to. Asking on installation is robust enough because parents buy devices for their children and set them up. You could make it even more robust if you required a cellphone store employee to do it. Whatever way someone is authenticated as a cellphone store employee would surely leak, but again: robust enough. An example of something not robust is asking the child upon access to the site. Brits tolerate ID checks for cigarettes, even though parents can buy cigarettes for their children.