Minecraft Account on Switch

Hi all. New to the group and all things Nintendo Switch. My kids got a Switch and Minecraft for Christmas. They're used to playing on Minecraft Pocket Edition (I think) on their kindle fire tablets and have Microsoft accounts related to that.

My question is how can I log them in on the switch with their existing accounts? When we open the game the only option is to sign in with a nintendo account. Presumably this won't relate to their existing Microsoft accounts? Any help appreciated 😊