How to gain administrator privileges on a kid's Android 6 phone?

I have an old functional Honor phone with Android 6 which would be perfect for the little ones to use basic apps such as Spotify Kids or Youtube Kids. I created a Google kids account, added it to my family and got myself a Family link app for parents where I can control the kid's account. Then, I have factory-reset the Honor and set it up with the kid's account. But now I am locked out. I cannot seem to find a way get parent's administrator rights. When I go to Settings > Security > Administrators I can only see an active parents administrator but the setting is grayed out. I cannot access it. Also, I cannot find any button or app that contains the entry to "parent access." What can I do? To add more complexity: I cannot install any apps because the Google Play Services are too old and you cannot update the services without updating them first ;) As administrator I would be able to manually upgrade the services so that I can restore Play functionally completely (I have done this before). But now with the kid's account I can no longer install apps from untrusted sources (fortunately), but it seems I can also no longer gain parent access. How to gain administrator privileges on a kid's Android 6 phone? Does anyone remember? Otherwise I might have soft-bricked the device and locked myself out forever.