Milepost guidebook

All of the messages so far are good info. IF you are VERY serious about driving from the lower 48, it would be money well spent going to the local bookstore (Borders, Barnes and Noble) and picking up a copy of Mileposts. This is the official Alaska highway guide. It is a mile by mile guidebook of the AlCan highway and its connecting roads in the Yukon and Alaska. The book also describes the four or five major highway arteries in Alaska proper. The guidebook lists mileposts of major (and some minor) points of interest, services, places to stay, eat and get car repairs.
Although the highway is much improved over the early years and most of it is now paved, it is still a long trip, over mountain range after mountain range, especially from Arizona. And it is by no means a superhighway. I personally wouldn't try an overland trip that far in the wilderness of that area with a six-year old. By the time you figure lodging and gas, it may well be cheaper to take the Alaska State ferry from Vancouver. The Milepost book, I believe, has a section on the ferry service and the costs. Try searching the internet for ferry info also. Plan well for success in an adventure like this, but by all means go. A trip to Alaska (and the Yukon and British Columbia) will reward you with memories that you will cherish for the rest of your life. What a beautiful place it is up there!! I must go back someday.
Just my thoughts on it. Good luck with your trip.