Minecraft Realms with my grandson with Xbox and Switch - safety concerns and other questions

[Stejay]

My six year old grandson is heavily into Minecraft at the moment, he plays here offline on my Xbox and offline at home on his Switch.  I was going to add him to my Nintendo Online family with the hope of getting a two player Realm so we could work together on the same consistent world on our relative systems.

 

If I sign-up for Realms, is there anyway to ensure that he only has access to our shared world in his Switch, or will it open it up for him to join any servers?  If he was to join a random server, work-arounds aside, are swear words filtered out in chat on the Switch?  Are there any other safety concerns I should be aware of?

 

Can you upload an existing offline world to Realms, and can you download from Realms too if I decided to stop subscribing at some point?

 

Finally, I know there are plenty listed online, but has anyone here got any good seed recommendations for us to start out with?