Social engineering is the most effective strategy, because iOS screen time controls are so buggy that eventually parents throw up their hands in exasperation and enable broader access than they would otherwise choose.

When everything is turned off by default, iOS Screentime is very effective. It also has effective tools for to grant certain exceptions, facilitated by Messages. It also distinguishes between "daytime" and "downtime" for the purpose of certain apps and app attributes, like the contact list. For example, we have ourselves, grandparents and the neighbors as "all the time" contacts but their friends as daytime only. They don't retain their devices at night but it is possible for them to pull them from the charging cabinet.