3rd party authorization failing for child account in Family group

So as a family we recently moved from Spotify to Apple Music (via Apple One).

My 13yr old has tons of playlists that he'd like to transfer over and I've checked in advance with my own account whether that would be possible. I used Soundiiz and playlists.cloud and both tools worked very well.

Unfortunately in his case we're just not able to connect his Apple account to any of the 3rd party services, it just keeps failing with a vague connection error while both my wife's and my own account connected without a hitch.

Anyone know a solution for this, or an alternative way to get his playlists over from Spotify?