You can't create a different island per user/account.
Once you have created an island on your console, all users have to share it.
Good luck if you have two or more kids...
Nintendo got away with forcing us to buy one game per user for years and years with Pokemon and Animal Crossing, and now they want us to buy one CONSOLE per user. They had the "excuse" of lack of memory space with cartridges, but with the Switch, there is literally NO GOOD REASON to do that but CORPORATE GREED.
That kind of practice shouldn't be allowed.
Seriously, shame on you Nintendo!