The kids might like to ride on a cable car (I wouldn’t generally use it as a mode of transportation but it might be cool if you’ve never been). It’s easy to get it on it at Powell & Market St (Union Square) and it can take you down to Ghiradelli Square and near the Wharf. Depending on where you’re staying, you could take Bart right to Union Square, shop around or eat and then hop onto the cable car. There’s not really a whole lot to do at Ghirardelli Sq. though, there are some shops & things & Boudin’s bread making factory is right around the corner. (It’s been a while since I’ve been there but I’m pretty positive you can walk from there to Pier 39 (Fisherman’s Wharf).