Google Family Link is Google's parental-control app for Android and Chromebook devices. (No parental-control software is perfect. Therefore, it's wise to also keep all your child's devices in your own bedroom every night, until the child is old enough to need overnight smartphone access.)
Google strongly encourages parents to use the Google Family Link to set up supervision. But, instead, I set up supervision using a Web browser.
I got a warning message:
"Your Google Account won’t work with some of your devices.
"You have 1 device that will no longer work with your account. Your Google Account will be signed out of these devices. You can’t use your account with these devices unless they’re updated to a supported version."
I ignored the warning, and clicked Next.
After finishing all the supervision set-up steps I could, the child phone showed a Google Play Services notification: "Account action required". Tapping the notification showed a 'please wait' screen, after which nothing happened.
Also, I was unable to view the full list of allowed and forbidden apps on the Google Family Link website. I saw a warning message: "[Child] isn’t signed in to any devices. Learn how to set up a device for [child]."
What to do?
(The child device is an old LG G5 running Android 8.0.0 "Oreo".)