How about Club Med on Hokkaido? The food at the breakfast, lunch and dinner buffets is awesome. There is also an indoor heated swimming pool, indoor heated jacuzzi, indoor Japanese sauna and an outdoor hot tub. Make sure you save your energy to stay up for the evening entertainment - the circus act is fantastic. The bar is included - all you can drink - and even has non-alcoholic options. The ski school is included for all parties.
Niseko and Hakuba are nice but with small kids, I would definitely go for Club Med in Sahoro, so much easier.