Back in March, Google launched 'Family Link,' a way for parents to create and manage Google accounts for their children (13 or younger). The feature ended up being very impressive, but had two huge limitations - it only worked on Android 7.0 or higher, and you couldn't use existing Google accounts for your kids.
Thankfully, Google is working on a possible solution for the second problem. The company has updated a support page for Family Link with the below message:
You can only use Family Link's parental supervision features with Google Accounts created using Family Link.
We've heard from many people that they'd like to use an existing Google Account with Family Link, and we're currently exploring solutions.
That's better than nothing, I suppose. I can understand why it might take more time to make that functionality work - in addition to the possible technical issues, Google has to follow all government guidelines related to online accounts for minors.
- Google Support