Things to do with kids

Please edit! This is only a start.

[edit] Sports

Exploratorium - really fun place
Academy of Sciences (in Golden Gate Park)- recently moved to this location with fancy-schmancy green roof and LEED design.
Randall Museum - a free-admission museum with an animal exhibit, exhibits that focus on local topics (like earthquakes), an outdoor lawn area with views, and crafts for kids.

Bring Your Own Big Wheel / Race Down Lombard St [1]
Sunday Streets http://sundaystreetssf.com/ Streets in different neighborhoods around SF get opened up to pedestrians, bicyclists, etc.

There's a really cool children's playground in Golden Gate park [2].
Lots of great playgrounds around the city. Most of them have the soft, spongy surface to break falls and give better traction when running. One of my favorites is Alta Vista Park in Pacific Heights, with its "fancy" sand, awesome views of the city, and lots of room to run around and have fun.
Glen Canyon, in the southern part of the city, is a great place to take children for hikes and other nature-related activities. http://en.wikipedia.org/wiki/Glen_Canyon_Park
Marin Headlands - take the walk path near the visitors center
Visit the Point Bonita Lighthouse (open Sat-Mon 12:30 - 3:30 PM).
Walk across the golden gate bridge, catch a cab back (bring the cell to call a cab)
Climb the stairs on the backside of Telegraph Hill. Start from Levi Plaza and walk up to the top of the hill. Several "streets" on this side that are only pathways, and even a flock of feral parrots (not to mention terrific views)