Our route is almost the same but we were there 5 years back. Around 1st 2 weeks of December. Kids were 9-14, they literally can enjoy the snow anywhere, just lie down and do butterfly flutters. But need keep the elderly warm and they will feel cold if they are not participating in the activities.
Tomanu Resort - We enjoyed. Kids took ski lessons, then we tried the different restaurants for breakfast, and within the resort there are also restaurants for dinner. Kids can also play sleds (self purchased) while adults can sit at an enclosed cafe space enjoying a warm drink. Also, go to Unkai Terrace on gondolas. Good experience, super cold, the kids play around, build snowmen while me and the hubby shiver in the cold. Lucky there is a little cafe so we got ourselves a warm coffee! Take nice memorable photos. Note that some of their activities are available from end-December or January only.
We drove coz we enjoyed the family chats during the journey. That time was our first time driving in winter, but we had always drove during travels. Be very careful and focused, do not jam brake as it will skid. Also must know which button for the anti-fog as window screen can turn foggy in winter.
The zoo, definitely a good experience if it's your first time there. We based ourselves in Furano, after Tomamu. Then we headed to an cozy ryokan in Otaru, while we also explored Otaru street and Tenguyama coz with the kids, it is bored for them if you get them to stay in the ryokan the entire day.
We also ended with Sapporo. Odori Park - quite bored for us. Shiroi Koibito very nice place to explore though we didn't buy the chocolates. Last day at Chitose airport coz we had an early flight home.
Enjoy your trip. In winter, the day is short. We will head there again but planning a different itinerary this time!