This might be far from ideal for others, but to fix it for my son, I got in touch with EA Support Live chat and got them to unlick his Xbox gametag from the broken child EA account. We then created a new clean EA account and linked this to his Xbox account.
Be warned though, this will reset the account back to Level 1. For us though, it allowed us to do something else I'd struggled to do which was linking his EA account to Steam so he can carry on with his progression on the ROG Xbox Ally he is getting for Christmas. So he is taking the hit with resetting to level 1 now, rather than at Christmas really.
I tried to do the same for my son, but the new account was immediately banned for an unknown violation of the user agreement. I tried to appeal the ban by stating what I was trying to accomplish and they stated that the ban was valid. The lack of support to get this fixed is almost comical. If anyone has any other possible solutions please advise.
