Welcome to the forum and TA, James ~
When are you planning this trip?
Your basic plan is a good one, and doable, it just needs a little adjustment here and there. Because BC can contain 4 UKs quite nicely, you may not realize yet how big BC is. And with children, there are other factors to blend in.
Here's a plan that you might find workable and which uses overnight places with some appeal. With Prince George, you will want to research thoughtfully where you stay. But it is predicated on a June-September travel time. All times are driving times, stops extra.
Vancouver > Whistler. 2 hours drive. 2 nights
Whistler > 108 Mile Ranch. 4 hours. 1 night
108 Mile Ranch > Prince George. 3.5 hours. 1 night
PG > Smithers. 4.5 hours. 2 nights
Smithers > Prince Rupert. 4.5 hours. 1 night. Maybe 2 nights because with the BC Ferries at PR, you will have to check the ferry schedule as it departs on odd # days in June, July and September, but even # days in August.
Port Hardy. 3 nights.
Port Hardy > Parksville. 4 hours. 1 night
Parksville > Tofino. 3 hours. 3 nights
Tofino > (BD ferry from Departure Bay to Horseshoe Bay) > Vancouver. 6.5-7 hours, including an hour wait for the ferry. 1 night
TOTAL NIGHTS = 15. This gives you 2 to play with. This itinerary is one that will allow you to have one full day to enjoy Whistler, one full day to enjoy the Bulkley Valley around Smithers, the wonderful things to see around Port Hardy with Umista at Alert Bay, the Parkville area with Strathcona Park, Cathedral Grove and several interesting attractions in that area.
Here's the map for all of that: http://goo.gl/maps/RvSHP
108 Mile Ranch: britishcolumbia.com/regions/towns/index.asp…
Smithers: britishcolumbia.com/regions/towns/index.asp… ... and ... tourismsmithers.com/index.php/things-to-do/
Prince Rupert: britishcolumbia.com/regions/towns/index.asp…
Port Hardy: britishcolumbia.com/regions/towns/index.asp…
Alert Bay: britishcolumbia.com/regions/towns/index.asp…
----- U'Mista Cultural Centre: tripadvisor.com/Attraction_Review-g499117-d6…
----- Tide Rip Grizzly tours out of Telegraph Cove: tripadvisor.com/Attraction_Review-g499151-d1…
(Courtenay, with its walk through the Courtenay estuary, might be a short stop on the way to P'ville: britishcolumbia.com/regions/towns/index.asp… ... and ... with its Paleontology Museum: tripadvisor.com/Attraction_Review-g181789-d6… )
----- Sand Sculpture Competition and Display: http://www.parksvillebeachfest.ca/
-----Coombs Country Market, with goats on the roof: tripadvisor.com/Restaurant_Review-g499125-d9…
----- North Island Wildlife Recovery Centre: tripadvisor.com/…REVIEWS
----- Cathedral Grove: tripadvisor.com/Attraction_Review-g154944-d1…
Pacific Rom National Park: tripadvisor.com/Attraction_Review-g154942-d2…
My 2 cents.
Edit: more posts while I was typing. The plan is good for August.
Edited: 2:15 pm, April 07, 2013