In California, bicycles are defined as vehicles and explicitly excluded from the definition of pedestrians. And operating a vehicle on a sidewalk is a big no-no, except to cross it to enter a driveway or alley. I always thought there was an exception for kids, and for adults riding with kids, but can't find a reference for that. In any case, unless there's a sign explicitly permitting bikes on a sidewalk, it is generally prohibited in California (municipalities can enact their own code for this). In fact the drivers handbook states that bicyclists must ride as close to the right hand side of the road as possible, but not on the sidewalk.

JB

PS - the prohibition of bikes on sidewalks must be one of the most widely unenforced sections of the vehicle code out there!