I just created a new google accout for our child (10+) so that he can log in to App Inventor.
The problem is that the site (appinventor) refuses to let you log in with an account that is managed with family link.
Conversely there does not seem to be any other login option here than with a google account.
App Inventor does not "refuse to let you log in with an account that is managed by family link." This is controlled from Google's end. You should be able to allow App Inventor using these instructions from Google: https://support.google.com/families/answer/7087030?hl=en