One word of warning, although the museums in Seattle are extraordinary, they are ridiculously expensive. Most adult tickets hover above $20 and the Space Needle alone is $28. I was relieved to find prices were half that for kids 12 and under. As if I didn't already have enough to worry about the girls turning 13…. It is probably worth getting the city pass but only after looking at the list of included attractions and deciding how many you are definitely going to visit. Mind you, if you get tickets online you usually get a discount so something to consider.
The other nice thing about the Wheel; you can combine it with a visit to the Seattle Aquarium just a few doors down. This has lots of interactive exhibits, touch pools teeming with sea stars and urchins and a few funny sessions where docents feed octopuses from a stick. It also has a diver speaks to the audience through a mike and is able to answer questions put to her by the kids. Overall, it is in that sweet spot of being big enough to get them interested (and maybe even sneakily teach them something) but small enough not to be too taxing.