Take them on a ride on the San Francisco street cars, along Market Street, Powell, Lombard, Van Ness, and other attractive streets.
After that, take them to Pier 39 where they can enjoy of a big variety of food, sweets, drink, and the sea lions.
During daytime, rent bicycles or roller blades and ride to the the Golden Gate Bridge. There a couple of bike rental booths that you can rent bicycles along Pier 39, Fisherman's Wharf and more.
At Pier 39, take them to the Aquarium, see all kinds of live sea animals, I'm sure they will love so much.
Past Fisherman's Wharf there's a pier with old ships, which you can take them there too, tour inside those ships, they'll like that.
Enjoy your day with them!