To clarify, are you able to sign into the Minecraft app on the Adult Profile?
If you are not able to, there may be an issue with the app itself. I would recommend contacting the app developer for assistance. They have provided an email address and a website as part of their developer info:
This isn’t a problem with minecraft or the Xbox app. Since my daughter has had her tablet she has been able to log into minecraft and the Xbox app and play fine. A recent update prevents the login. When you try you get to a part where the app directs you to a live.com address and the fire blocks it. It’s a new restriction from amazon preventing this from working. Trying to add live.com to the allowed website does not work
My kids also have minecraft on their xbox and Switch and can login fine from them, I agree with doddy that the restriction looks to be on the amazon parental control side that is stopping us from logging in. You can see the browser open to the login.live.com URL, but then I get the this is a blocked site message and it is not possible to add that to the allowed websites in the parental control.
I see the problem firmly in the hands of Amazon as they are doing the blocking and not the third party.