Issue logging on device with samsung child account

[Bipsen]

I have created a Samsung child account via the family function - she is 11 years old.

When trying to log her on - on an old Galaxy S9+ phone, I just get an error (in Danish) "Behandling mislykkedes" - which probably corresponds to something like "Processing failed" ...

How do I successfully log her on ? I need to configure the account/phone, in order to configure a Galaxy Fit 3 for her...

[Bipsen]

Did some investigation - seems like the S9+ only have OneUI 2.0 - and minimum 6.0 is required in order to support child accounts ...