Vancouver is a great place to bring children any time of the year, but particularly in summer if you like the Great Outdoors.
They can explore Science World, the Aquarium, The Space Centre and local museums.
Outside, they can go for beach or forest walks. Even in the rain (they get more rain than Seattle), walking in the woods can be fun.
Take a seabus and ride on the cable car up to Grouse Mountain or Cypress Mountain.
If they prefer water, they can swim in one of the many local indoor swimming pools.
Visit the parks, go for beach or forest walks in the sun, skateboard at a local skateboard park, attend the International Children's Festival,
Visit one of the indoor kids' amusement centres.
In summer they can enjoy the giant outdoor pools at Second Beach in Stanley Park or Kitsilano Beach, go to ballgames at Nat Bailey Stadium, watch spectacular fireworks
They can hang out at KidsBooks or the public library (on strike at the moment but hopefully will be sorted by then!), visit the Children's Market on Granville Island, walk across Capilano Bridge and do the tree walk
Play Miniature Golf ................
Long flights are a bit of a drawback, but I guess you would have those in California also