The cable cars are worth riding, at least once (and parents with toddlers will probably ride them over and over). Of the three routes, Powell-Hyde (to Aquatic Park) and Powell-Mason (to Fisherman's Wharf) are the most scenic, and the easiest to catch from downtown.
Board at the beginning of the circuit, or hail the car at a stop marked with a brown-and-white sign. Wait for the car to come to a complete stop before boarding. Children must ride on the inside.
Even if you don't have time for a ride, kids love to watch the cable cars turn around at the end of the line (marked on the map)