1. The Needle: The best time to visit is on a nice sunny day before sunset and stay till dark so that you get both a night view and sunset view. If your child wants to visit the needle, you might consider making a dinner reservation to get the free deck visit. Here is how the math works (assuming they don't require a minimum spending to get the free deck visit - you may need to double check when you call to reserve): The deck visit is $8-16, depending on age. If you order the cheapest items on the menu, i.e. appetizers and soups, you might actually come ahead. Restaurants at these places as you know are never cheap. But if you order, say, clam chowder at $9, you'd get the soup and free deck visit. Again, be sure to ask if they have a minimum spending requirement. Here is their lunch menu:
http://www.spaceneedle.com/restaurant/menus/lunch.asp

2. With 7-8 year old kid, consider visiting Ballard Locks, good place for kids and adults
http://www.nws.usace.army.mil/PublicMenu/Menu.cfm?sitename=lwsc&pagename=mainpage

3. Another good place for kids and adults alike is south Lake Union - Center for Wooden boats. It's a place with a nice city view and activities for kids. Kids also might like the float plane takeoff and landing right on the lake next to the center. On Sundays, they offer free sail boat rides on the lake (reservation is needed)
http://www.cwb.org/

4. Pike Place Market: Longest running public farmers market in US, celebrating centinial this year. Kids might like the colorful pigs and the fish tossing.

5. Olympic Sculpture Park: brand new this year. Great place for beautiful scenery and modern art in park setting on water front.

6. Boeing Factory in Everrett: watch airplanes to be assembled in the largest factory building in the world. Not for everyone, but kids might like and also educational for kids. Could also visit Boeing Museum south of Downtown Seattle.

7. Ride the ferry from downtown Seattle across Puget Sound to Bainbridge Island and extend your trip further to Olympic National Park, time permitting (you need at least 2 days or more to fully enjoy Olympic NP)

8. Northwest is very different from San Francisco. For one thing, you don't see snow-capped mountains in summer (or any time) in SF.

9. There are many many more places to visit outside the city such as Mt Rainier (road is a mess this year), Mt Baker, Olympic National Park, N Cascade National Park, San Juan Islands, Mt St Helens, etc. 4 days are not enough to visit these places, as they each require at least 1 day and some longer.

Enjoy your trip!