I highly recommend the Wii especially if any of the kids are under 13. I have Wii/Playstation and xbox as I am a bit of a gamer but when there is kids around its the Wii every time, in fact even my kids Grandparents love the Wii and join in. Last Christmas they spent hours playing the Wii with the kids and even when the kids went to bed they stayed playing half the night. Over 13 and you should look at the Playstation/Xbox
My tips would be if there are 4 kids then you will need 4 controllers, you can get away with 2 but then the kids will fight over them. Plenty of bundle packages around both new and 2nd hand.