After poking around a bit, I was able to get my kids set-up with Minecraft cross-platform multiplayer.
See the link in the previous paragraph for details.
Once you set it up, and add your kids and their friends, they’ll be able to join each other’s games through the Xbox Live service for free (though accessing it via Nintendo Switch requires their paid online service “Nintendo Switch Online” for access).
Once everything is configured according to the details of that article, you’ll need to be able to easily share links to the relevant Xbox Live accounts.
Here’s an example:
You can just modify it by deleting “Sample%20Gamertag” and adding whatever gamertag you wish. Be sure to replace any spaces in the gamertag with “%20” to ensure that the link is URL encoded and stays intact when pasted into emails and various messaging apps.
Asking friends to add your Xbox Live account via a link allows them to do it from any device and also coaches them to get logged in with their own account first before they can add yours – which is a great way to help them figure out what their own XBL account is since not everyone knows off the top of their head (especially when it comes to parents setting up accounts for their kids).