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There's a cool little water taxi/ferry that operates across False Creek to Granville Island. It's almost like a big toy (but perfectly safe), so the kids will get a great kick out of this. It will be an adventure for them. It's only about 10-15 minutes and just a couple of dollars, so doable.
At Granville Island there are tons of shops and stalls to explore. You can pick up a lunch here and have a picnic. Granville Island also has some kid-oriented activities. There are also a couple of theatre companies here. I'd check out tourismvancouver.com to see what's on during your time frame. At the end of False Creek there is a children's museum. Sorry, I haven't been in, but it's a consideration.
If you have a vehicle there's a very cool Museum of Anthropology at UBC. That may seem over the heads of the kids, but it has a collection of totem poles, and sometimes in the workshops outback there are native people carving new ones.
Vancouver is a very active city. It's kid and pet friendly. And there are a couple of sand beaches right in the city (English Bay and Sunset Bay Beach), plus the massive Stanley Park. So you have fresh, active options in the city.