How about San Diego Melly? They have Sea World there and Legoland and the weather is nice.
I'm from San Francisco so you could do Great America which is Santa Clara (45 minutes away from SF) or Six Flags Animal Kingdom (formerly named Marine World) but they have big rollercoaster rides. There is also Santa Cruz beach boardwalk
(which reminds me of the Jersey Shore) and that is about an hour and half from SF. SF doesn't have any theme parks in it per se, except when the fairs come around to the area. I think the only fun thing they have for kids would be like
the Blue & Gold Fleet tour (its on a boat around the bay), Alcatraz island, Angel Island, or the Shark Experience at the wharf. Its basically a concrete jungle unless you cross the bridges to the east bay or south bay.