It's totally doable to block YouTube with pihole, and also to make it blocked only on certain devices.

The regex are: (^|\.)youtubei\.googleapis\.com$ (^|\.)ytstatic\.l\.google\.com$ (^|\.)ytimg\.l\.google\.com$ (^|\.)youtube-ui\.l\.google\.com$ (^|\.)youtube\.com$ (^|\.)ytimg\.com$ (^|\.)googlevideo\.com$

You can create groups and assign devices to them, and assign the block rules only to certain groups.

The only annoyance with this is that it blocks logging into Google since they redirect to YouTube to set a login cookie as part of the Google login process. If you're already logged into Google though, everything works as normal, and you can always disable pihole for five minutes if for some reason you got logged out and need to log back in.

My kids figured out disabling Wifi disabled the Pihole within hours, and that was when they were ~9. They are intelligent opponents and a very fast moving target.