We took our kids to San Francisco in April, 2004 when they were 7 and 8 years old. We all had a wonderful time! There are many things to do with kids:
Pier 39 - sea lions. Kids can spend a lot of time watching the crazy antics of the sea lions! Lots of them are near the pier and they are so funny. Pier 39 is also a fun place to walk around, shop, eat.
Fisherman's Wharf - kids love seeing the day's catches. Overall fun place and a must for kids.
Alcatraz - we took the kids there and they loved it. They thought the old prison was cool (they got to go in some of the cells), and they loved running around the island and looking at things.
Coit tower - they liked the view of the city and climbing the tower.
Lombard street - I think that's the name of it. The most crooked street. We had to drive down it several times! We also drove down the steepest street.
Ghirardelli square - get some chocolate!
Ride the cable cars - a MUST! Also go to the museum and powerhouse.
Exploratorium is a great museum for kids.
Chinatown - good place for buying cheap souvenirs. Has a whole different feel to it and the kids liked it there.
Muir Woods - this is a little bit of a drive across the Golden Gate bridge. Very, very pretty area for hiking. I highly recommend it. There was a very nice little loop trail we did through the redwoods. It gave the kids a chance to see redwoods and giant ferns.
That's all I can think of right now. There is a lot to do in the San Fran area and the weather should be okay in March. Good luck!