Lots of good information in this thread so far. I went to Niseko a month ago and I'll add a couple more thoughts. 1. Domestic flights within Japan are usually much cheaper from Haneda than Narita. Air Do and ANA are domestic airlines that go everywhere with good service.
2. Traveling with a child, if you do go to the Niseko area I would recommend taking the bus from Chitose airport to Niseko...make reservations. We didn't and ended up taking the train, which was beautiful, but harder with a kid in tow.
3. Food, you will be able to find Western food if the little one is particular, but there are a lot of japanese foods (such as ramen and udon bowls) that he or she will probably love.
Have fun planning!