Your cheapest, easiest transportation to the Vancouver Pan Pacific would probably be to take the QuickShuttle (
http://www.quickcoach.com/ ) from the Best Western Executive Inn, if you are staying downtown, or from the airport, if you are staying near the airport. The Best Western Executive Inn is near the Space Needle, and the QuickShuttle will drop you off directly at the Pan Pacific. If you're leaving for Vancouver directly from the airport, the QuickShuttle is definitely the best bet, although you will want to get snacks or take-out meals and bottled drinks, because they do not have food onboard and it is a long ride.
Amtrak is a beautiful ride, from everything I've heard, and, if you don't have much luggage to haul, you can probably take the light rail from downtown to the station. If you have a lot of luggage, don't even try it--take a taxi. You'll also need to take a taxi ride to the Pan Pacific when you reach Vancover, but as I recall, it's a relatively short trip.
However, if you have a person traveling with you in a wheelchair, I would strongly recommend the QuickShuttle over Amtrak. I have never had a good experience riding Amtrak in my wheelchair, and they will sometimes split up your family, if there are more than two of you. They won't do that on the QuickShuttle.
As to your 8-year-old, I'm not sure if there'll be snow in Whistler during the summer, but it is a beautiful area.