This issue might not be related to Stadia directly, but definitely impacts it. The culprit seems to be Gmail which makes it impossible for you to create an account with Bethesda or Bungie using a "child's Gmail account". Both Bethesda and Bungie return an error that prevents their account creation using these types of child Gmail accounts. I have also tried linking it directly without account creation and this has failed.
Why is this bad? My child cannot play Destiny or Elder Scrolls online on Stadia without linking these accounts despite them being allowed in Stadia family controls.
Others reporting having this issue with no solution found:
