Cannot specify child account as Primary User for HomePod
Both my kids have HomePods. My 12 year-old is specified as the primary user on his HomePod for Music/Podcasts in the HomePod Account field. My 9 year-old used to have this same configuration on her HomePod but after I changed my password (as the My Home account holder) she can no longer sign in. I receive the following error message: "Verification Failed: This Apple ID is not supported on this device." OK is my only option.
I have signed in and out of her device. I've reset the device to factory settings. I've set her brother as the primary user on her device successfully. But after none of these steps can I sign in as her. I recently bought a HomePod for my own room and cannot sign in as her on that hardware. I've tried these steps on software version 15 and now with 16.1. This leads me to believe it is the account. Any ideas?
HomePod mini, 16