A lot of families come to Niseko. They have excellent International ski schools here for children 3 years of age and up. Our preferred and recommended ski school is NISS ski school. And they also do half-day skiing lessons for kids, morning and afternoon. I'd recommend half day lessons for your children, unless you want the whole day to yourselves to ski that is. Niseko is reasonably cold, around -5 to 0 degrees in the village and -5 to -10 degrees on the hill. So good quality outwear is essential as well as goggles. Sunglasses are no good during Dec, Jan and Feb.
There are a lot of restaurants and bars in the village as well. Also shops but not what you'd call a shoppers paradise.
If you like powder snow then this is definitely the place to come. Niseko is up there in the top resorts in the world for snowfall. 13-15 meters per-season. But keep in mind due to high snowfalls, sunny days are few especially during Jan and Feb.