u hv big family, it is actually cheaper when u evenly divide the cost
and hokkaido itself is actually more convenient with own transportation, except all those big town

my opinion, the elderly also hv more time to rest... less walking/waiting

there are many website... u just compare the price
http://www2.tocoo.jp/en
http://www.toyotarentacar.net/english/
http://nissan-rentacar.com/english/shop/hokkaido/

ps: non-peak season, no need book... many car rental hv counter at airport

7 seater car sure will more expensive than sedan. Those car rental shop all have standard price. If cheaper also wont more than 1500yen different.  My fav car rental are Nippon Rent A Car company.

D1 super rush....... i can said that u will reach Ashikawa hotel at 8-9pm even u may just touch and go at tourist spot
D2 another log hours inside the car (4-5 hours)
D3 No problem
D4 Summer at Niseko.....not much attractive
D5-D7 No problem

U r welcome, just my opinion for your plan

Day 1 - Arrive in Chitose / Furano / Biei / Overnight in Asahikawa
Day 2 - Noboribetsu / Lake Toya.  I will suggest you all to revisit Biei/Furano at the morning again until you all really satisfy.  I have been visited Biei/Furano twice (each visit I stayed three nights here)......so many attraction.  The best time to visit blue pond is with the strong sunlight which will give your photo "wow" effect. So u may reach Noboribetsu late evening or nigh time.
Day 3 - Hakodate........Enjoy your day time and Noboribetsu, visit Lake Toya, Lake Onuma, overnight at Hakodate
Day 4 - Enjoy day trip at Hakodate, stay at Hakodate
Day 5 - Shakotan Peninsular / Otaru There is a fruit orchard near Otaru, u can go there after visit Shakotan
Day 6 - Jozankei Dam / Overnight Sapporo
Day 7 - Sapporo City / Overnight in Chitose
Day 8 Fly Back ..

Hey there, hope this help ( just came back from Hokkaido self drive) 

1. for your D1 which route you plan to use? If same as mine, you will be reaching Asahikawa first, so you will need to drive to Furanu/Biei and back to your hotel (1 hour+ one way) Or the other route, you will be driving a lot more hours. How about staying in Furanu instead? Suggest you stay at least 2 night in Furano (if you are into Flowers, melons, lavender, country side and mountain)
2. Noboribetsu and Lake Toya are nothing special, worth a detour but not a full day, you may visit them while on your way to Hakodate.
3. Spend at least 2 nights at Hakodate, you won't regret it.
4. Skip Niseko
5 Day trip to Otaru but no need overnight. Stay at Sapporo instead. Sapporo to Otaru is just around 45 minutes. Spend 2 nights in Sapporo
6 Every souvenir, chocolate, or Hokkaido limited stuff are available at the Airport at around the same price. So, don't worry if you forgot to buy anything.

too rush 
all the place is totally out of order...   
waste traveling time
dont think u able to reach hakodate
u think hokkaido is very small? 
at the end u only hv 8days... u sure u have enuf time??

...unless this is your plan for full day driving trips (minus enjoying the trip) 
stamping all the way
---

after asahikawa, u should head to sapporo -> otaru -> jozankei -> ?? hakodate -> lake toya
noboribetsu is just below chitose, u better make it on last day

pls use google maps to plan your itinerary, at least u should learn the traveling time 

Both also fail plan. Have to see Hokkaido Map before make such speechless waste money plan.

Proper Plan should be
Day 1 Chitose > Furano > Biei > Asahikawa (overnight)
Day 2 Asahikawa > Sapporo (lunch) > Otaru > Shakotan Penisula > Otaru (Overnight)
Day 3 Otaru > Niseko > Yakumo (Lunch) > Hakodate (Overnight)
Day 4 Hakodate > Hakodate (Lunch) > Noboribetsu ( Onsen + Overnight)
Day 5 Noboribetsu > Sapporo
Day 6 Sapporo > Jozankei
Day 7 Sapporo > Chitose (Overnight)
Day 8 Fly Back

Please Key in all placed in Google Map, It make perfect Circle, Making Sense.

ok... ur plan ur decision...
seriously LEARN to plan itineray with google maps

How to Plan a Trip Itinerary Using Custom Google Maps

Some sample i create
Google Maps

platingirl gmaps driving duration in Hokkaido a bit underestimated one. Just one comment from me. Other than the expressway, roads are typically very low speed limits (40-60km/h).

Dear Lucaswjk and RegentCid,

Relax, brothers ....... If you refer back to the her first thread, I just slightly modified it to enhance her travel experience.  She going to take off soon, I cannot simply suggest her to make new booking for her stay as u all know that:

Summer is a peak season for Hokkaido tourism, last minuets booking may be very expansive.
We do not how many travelers join her and who are they (adults, elderly, children or infant)?  My own experience to make booking at Furano guest house (travel with my six months old infant) was a very challenging task, not every guesthouse will accept children and infant.

Since she already explained her reasons for her planning, we give her input to enhance it.  Thank you.

u can try japanese style room, tatami type (-->sleep on the floor)
normally hotel allow you to add futon

did that... 

in this forum they hv special status 

actually i hv been telling alot of ppl to planned their itinerary using google maps
but nobody do this, at most i saw excel sheets

taking this chance to show the power of googlemaps
let them see for themself the usefulness

If you booked business hotel liked Toyoko Inn, Super Hotel , Dormmy Inn, they usually allow one child (may be below 5-6 years old) free to sleep by using the existing bed.  If you have two children age more than 5 years old, you may need to book two rooms.