"It found that the “family pairing” scheme, which gives an adult control over a child’s account settings, did not check whether the adult “paired” with the child user was a parent or guardian."

Chapter 8, Article 1 of the GDPR[1] states:

"Where the child is below the age of 16 years, such processing shall be lawful only if and to the extent that consent is given or authorised by the holder of parental responsibility over the child."

This doesn't seem vague at all.

[1] https://gdpr-info.eu/art-8-gdpr/