some suggestion / comments
from arrival at Chitose, drive anti-clockwise
1) Furano/Biei - take the Doto Expressway from Chitose.
Stay 2 nights, take your time to explore the area
2) Asahikawa - r u planning anything in Asahikawa ? If not, can skip if just using to rest .. it's a city, nothing much IMO. Zoo for kids ?
3) from Furano, can go straight to
Otaru - the drive is slightly long using non-toll road but it is fine.
- stay 2 nights (Sankaku Market / Canal night view / Sakaimachi Street + 1 day trip to Yoichi - Nikka Whisky/Fruit picking or Cape Kamui for views)
4)
Sapporo - up to you. Having a car is not too bad, can go to the out-skirts .. many parks around
5)
Chitose - Staying at Chitose Airport hotel ? or your Chitose hotel provides Shuttle to airport ? How early is your flight out ?
Airport Hotel is not a bad idea as the domestic terminal is worth exploring for 0.5 days ... albeit more expensive.
Check the car rental operating hours if you are returning late evening/early morning.
Driving between these locations in your itin is straightforward. Non-toll roads are pretty good. For peace of mind, you can consider the HEP since you have a big group. Not having it is fine as well for your itin.
A lot of driving resources available :
Hokkaido driving handbook
https://en.visit-hokkaido.jp/library/brochures/pdf/driving_en.pdf
map of Michi-No-Eki
https://www.michi-no-eki.jp/stations/english
Toll Charges
http://dc.c-nexco.co.jp/dc/DriveCompassDiagram.html?ca=hokka
I usually rent cars from Jalan.net or Rakuten Japanese sites which I find are cheaper. Jalan has JPY 10k coupon now.
I have never done road trips with young kids before. I wld probably minimise number of hotel changes.
Hope others chip in on the sights for the various destinations we well or places to visit. Noboribetsu would be interesting too for 1st timer.
What are your interests ? food, scenery, shopping, nature, onsen, relax time ?
Hokkaido has a lot to offer ... dont think you will end up with nothing to do
Thanks thwysg for the vote of confidence ... i have only done a few trips, mostly Hokkaido, but had researched a lot more than needed ..