I had the same problem.
I read the instructions from this thread and created a new EA account for our kid at ea.com with browser, but using that from PvZ GW2 didn't work, although we were able to login with the same account to ea.com with a browser.
The problem was solved by creating yet another new account from the PvZ GW2 game itself, when the game requests to connect the X-Box user with an EA account, and there's an option to create a new EA account in the game app in X-Box. Note: The game requested "your parent's email address" when creating the new account, so that may have also played a role here.
This is still horribly broken. Created an ADULT account so my daughter could play Anthem with us and now we can’t login at all. EA is enforcing some age rating despite me unblocking this on both the Xbox and EA side.
