Not related to Xbox, but hoping someone can help!

This is seriously the most straight forward answer I've received and it worked! I connected him to our Nintendo Family Account and I can now connect him to his Microsoft account. THANK YOU SO MUCH!

Thank you for taking the time to post this on the Xbox Forums. We understand that you are currently inquiring about how to sign into your Microsoft account on your Nintendo Switch console to play Minecraft on a Realm.

You can only sign into your Microsoft on titles that allow this, not on the actual Switch system. This is the case for Minecraft. The Minecraft
for Nintendo Switch FAQ actually provide instructions on how to sign-in, which I will post here:

a. On the Minecraft game home screen, click “Sign in with a Microsoft account”
b. It’ll bring up a screen to go to: http://aka.ms/remoteconnect with a unique 8 digit code
c. Enter the URL on another device (PC or Mobile phone)

d. Follow the instructions on the website by entering the code and complete signing into your Microsoft account.

Keep in mind that you might need to configure your child's Privacy and online safety in order
for them to use certain social features, such as joining Realms.

I hope this is of some use to you, Ash.

Thank you for taking the time to reply to this thread, Ash, and let us know what worked for you.

As you have found a favorable solution to this matter, we will now be locking this thread.

If you have any other questions or concerns that differ from this matter, please, feel free to create a new thread and we will cater to you there.

Thank you for being part of the Xbox Family and we wish you a wonderful day!

You must make a Nintendo account. It also may be more useful to visit the Mojang fourms.

For this and any Minecraft issues you might have better luck on the Minecraft forums or the Mojang
support for better assistance