In San Francisco, Golden Gate Park comes to mind as "child friendly". If you have your own transportation, Children's Fairyland in Oakland is a favorite place for young children, where all the fairy tales and nursery rhymes come to life. (It's an old park, been around for about 65 years or so, but has been revamped. I still have my magic key, from when I was a child.)

Alcatraz Island may interest you. If you go to book a tour, do so through the NP concession rather than any other online place so that you get the best deal.

Donna