First destination is Asahikawa, second largest city in Hokkaido. Tourist attractions are Otokoyama sake factory, ramen village etc. Ashiyama zoo is good for kids. Famous penguins parade there during winter time is essential. Then drive to Daisetsuzan national park. See Ginga & Ryusei waterfalls and enjoy HYOBAKU ice festival if visit late Jan to Feb.
Overnight in Sounkyo

Drive to Biei town, and see Aoi ike-blue pond and Shirahige falls. Scenery is very different when visiting in winter there. Then go to Furano, cheese factory and Tomita farm(lavenders maybe available in the greenhouse) are options. Why don’t you try dog sledding before check in Tomamu resort in the afternoon today?
Overnight in Tomamu

Tomamu is popular winter resort. Many winter activities available there, for example snow mobile, banana boat, snow rafting, snow shoe hiking, snow camping etc. besides ski and snow board. After great experience, taking express way back to Sapporo. It takes about 2 hours.