Nintendo rules 2-4 player local couch co-op. I've used my Wii U 1000x more than my Xbox One. When I want a higher end game, I play on PC. But of course PS4 Pro is a good choice too. However for younger kids, you can't go wrong with Nintendo. Even for adults, we have had tons of fun playing FIVE players at the same time on Mario (one to spawn jump points and such with the main controller).
There are barely any titles for the Switch yet, but that'll change. I would also consider buying a used Wii U with a whole bunch of games and controllers on Craigslist or Ebay + the new Zelda, of course.
I'm buying the new Zelda for Wii U just to rip it and play it via emulator (once it works). I really want the highest graphics potential, and the Switch apparently isn't that much better graphics-wise than the Wii U. It does Zelda at 900p instead of 720p, but it's still at 30 fps which is unacceptable to me.
Hence using an emulator. I played Mario Kart 8 at 4K60 and it's insanely sharp and fluid. I'm just glad the new Zelda is out on Wii U, mostly to play it the way it's meant to be played: On a PC
I should also recommend this: Buy a projector for gaming. It's super
awesome. Especially with 4 player split screen titles, on my 138 inch wall-sized projected image, each player's quadrant of the screen is literally bigger than most people's main TV. And that matters a bunch. But for games like Mario where everyone shares the same screen, it's a total blast. Why do kids always tend to go right up to the TV to play games? To see it better, and be more immersed in it. They are natural video enthusiasts. What's better than a HUGE screen to play on?
And I'd recommend a DLP for gaming, it's got super smooth / fluid motion. Make sure if you do go this route you consider noise and input lag too, both are considered deal-breakers. My only complaint about my w1070, apart from the meh contrast, is the noise.