Family sharing with minors

Since two months I pay for an Apple Music family subscription.

I cannot add my children to it so as to enable them to use Apple Music.

My children, 12 years old, own Android devices, and they downloaded and installed Apple Music.

I went through system settings on my MacBook Pro, and initiated an invitation.

When they receive it, I fill out all fields as required, but always end up at "An error has occurred..." (in German). There is no concise German guide.

Where do I find a concise step-by-step guide how to get this working?

A frustrated dad.