The WiiU should do nicely for 5-8 year olds. There isn't a lot available for that age group on the Xbox one or ps4.
Well, unless you're going to buy a ton of games, I'd absolutely go with an XBox One or Playstation. We got the XBox one for Christmas and both the kids love to play it. We started off with Disney 2.0, which has a create mode (think Minecraft, but not just cubes of stuff) for all kinds of custom playhouses. I think it came with a Dance-off sort of game as well. Then we grabbed a few Lego-based games - Sam's favorite is the Marvel Super Heroes. His latest acquisition was the Skylanders Trap Force, which is a lot like Infinity where you have to buy the characters to play them. This has the added element of buying traps to capture bad guys in - then you can play the bad guys in the game. I've heard good things about the PS4 as well - I don't think you can go wrong with either of those. You're also coming in at a time where ou might be able to get the games/system/figures for a lot less than last year.