Can't link my sons google play account.

Can't link my sons EA FC Mobile game to thier email via Google Play, I don't want them to loss thier progress if they switch to a new tablet. 

This is the error it tells me on both tableta, they are under family link, but both are 13yr or older. They don't have Facebook, so I can't try to do the linking via Facebook.

I have cleared cache, cleared data, uninstalled it, still nothing works.

Please help.