Our kid has my old Pixel 5a running #GrapheneOS with no physical SIM. It's basically a big music player/Meshtastic messenger. We just installed a silent.link eSIM to test it out. It was about 22k sats for setup and $5 worth of data. At our local rate, it should be about 3GB. A quick Threema video call burned about 5MB.
The data doesn't expire, so it should be handy for some situation where he might need to send us a message. I'd rather not have any network connection on it at all, but this seems like a decent compromise. 3GB of data on today's internet doesn't last long, and you couldn't squeeze 5 sats out of him with a steamroller. He will NOT want to pay for a reload. It should be self-policing for those times we're not there to supervise.
Overall, this is probably a fair option for a kid phone. Pay as you go, just like the old days. Anyone can reload it from anywhere, also. I can't see any reason not to just have one loaded up if you have the eSIM available.