You've seen the sites made by adults claiming to know what kids would like in SF, but as a child who lives in San Francisco, I know the truth. I can tell you from personal experience what would be fun for a kid in SF.
The Fillmore is a really fun and busy place that has lots to do. You can hang out at Alta Plaza Park Or shop a little lower down. There are also a lot of delicious resturants and food places!
Yoppi Frozen Yogurt in the Fillmore