I love the idea and it’s something I thought about doing too, so I’m super interested in this thread.
For me, I thought it would be interesting to remove the existing stereo and mount everything it the double-DIN hole left behind. That would give you access to power, ignition, and speaker wires easily. You would need to do some rewiring between cars, but it would be the most practical spot.
From working on some fleet vehicles before, I’ve seen interesting systems where all of the accessories were on their own battery that only charged when the car was running or off an external charging cord that could be connected if it was going to sit for a while.
The benefit of a separate power system was that the equipment didn’t need to boot up every time you started the car. Maybe the display and anything else could be powered only when running, but if the main computer was always powered that would save time. It could also get a signal from a battery maintainer that the accessory battery is low and perform a graceful shutdown.
I run AdGuard as an addon to Home Assistant. If you want to stick to AdGuard you can go to Settings > Client Settings and set a per-client filtering rule. There is a tab in the client settings that can be used to filter specific services with a click. Setting an IP reservation for your child’s device in the DHCP settings of Unifi or your router will help ensure the IP address stays consistent.
For the PiHole option, you don’t actually have to let the PiHole handle DHCP. You just need to tell your DHCP server what DNS server to use. For example, my router is .1 and my Raspberry PI is .2. The DHCP server, my router, tells all devices .1 is the gateway and .2 is the DNS server. You may also need to set these settings on the individual devices to prevent them from ignoring your DNS settings, but that can be done from the network settings. Avast had some safe networking “feature” that would force my DNS settings to be ignored, same with my Android phone.
Of course, the downside to all of this is that any different device, different IP, device from a friend, or mobile data could bypass these restrictions. You may soon be in an arms race with your child and chances are they can get more clever that you can in a shorter time frame.