It doesn't work with a single joycon that I'm aware of. The best way to play it is with gyro which basically acts as a 3rd joystick. It's best played on a pro controller. Despite the kid friendly visuals, the game has relatively complex controls. A lot of so called pro gamers on here still can't even figure out how to use gyro apparently.
The game has a short-ish campaign made up of small levels that act as tutorials mostly, but they're fun. Then there's a great co-op mode, and multiple vs modes. The vs modes do have a lot of skilled players. If you wanted to only play single player content, the Octo Expansion is totally worth it, but it ups the difficulty.
I'd say it's worth it if you also want to play it, because it's possible a young kid may not be able to.