Another vote for Vancouver. I'm not sure I'd combine Vancouver with a US city. It can take a while, and be a bit of a grind, to get from downtown Vancouver to downtown Seattle, especially if the border is backed up.
Also, (personal preference here), I'd skip Victoria and head up to Tofino/Ucluelet on Vancouver Island with a 5/7 year old. The beach there at the Pacific Rim National Park is very cool. However, you'll also find plenty to do just in Vancouver.