Why can't I block YouTube during downtime on iPhone?

I've been reading posts for years and it is just maddening that Apple, one of the most successful companies in our country's history. Cannot make a reliable function to block all access to apps and internet during down time. I've done all the tricks listed here, updated software, rebooted phones etc. My kids can access YouTube through Safari or directly through the app any time they want during down time. Yes, the "always allowed" is set up properly. Why can't this work Apple?

[Re-Titled by Moderator]