Wo, Wo, Tim. Don’t take offense if someone doesn’t accept your answer.
I do not. This is not my first rodeo. Believe me. 
I am simply expressing my thoughts in the context of your “it appears no one who has commented has actually performed this action” tone. 
We don’t need to have personal experience with a situation identical to yours 100% in order to try to help you. Many people work here and deal with network aspects for everyday and various solutions and their implementations are nothing new to them.
I understand that you want a solution that will suit you 100%, but there is a risk that you will not find one and either you will adjust yourself a little to the situation or someone will adapt the solution to you, probably for $. We can only point you to the potential direction, but the rest is up to you …
Most commercial Parental Control Apps and Devices are based on direct end point control. These gateway based solutions are Usually just domain blacklists. So more or less the same as what you do with Pi-Hole.
Some offer hardware / application combinations per device. But you don’t want an app per device.
https://www.topattack.com/list/best-free-parental-control-software-review/81
Or you could think of a dedicated Windows machine and Comodo Firewall… where you will introduce traffic controls for the network segment. If you prefer windows and gui.
I hope you will find a solution that meets your expectations and you will be satisfied with it.
Peace.
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of course, once the child becomes smart enough to press the ‘reset’ button on the router, all this is moot. All that time and effort wasted.
Because it is a fight with the wind …
The child will always have access to the Internet in 2020, even when away from home, at a friend’s, on the phone. This fight cannot be won in the long run.
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