Kindle Fire tablets will not allow Microsoft log in

I have three Kindle Fire tablets with Free Play for my kids. For over a year they were all signed into my Microsoft account and they were all able to play at the same time. About a month ago, the tablets were all signed off and whenever I try and sign them back in I don't have the option to pick an account and I get an error. My Microsoft account still works on the computer and Xbox, but I tried creating new accounts for the tablets anyway. However, the sign in screen never comes up. Microsoft and Xbox support sent me to Mojang support who sent me here.