We went to Karuizawa for ski. It is very recommended if you go with kids! Because….it takes just 1 hour from Tokyo station by Shinkansen (express train) without connection. When we arrive in Karuizawa, the prince hotel is just 10min by walking. (take a taxi if it’s too far for you)
I was not sure about staying in a cottage with the kids, but actually it was just perfect. They have “Kids park” for small kids who doesn’t do ski yet. They can enjoy sledging!