My two older boys were 9 and 7 when we visited LA this past November. Here is what we thought:
Hollywood Walk of Fame: We actually stayed at the Magic Castle which is a 5 min walk to there. We went to check it out and was crazy busy. We didnt even enjoy it. On our last day we walked down around 7 am to grab coffee at Starbucks. There was hardly anybody around, it was awesome! My boys were able to see all the stars and get pics infront of the Chinese theater and Jimmy Kimmel studios with nobody else in the pic! My boys liked the Walk of Fame more than I thought they would.
La Brea Tar Pits: Hands down my 9 year olds favorite place. He loved the tour and seeing the tar, as well as the museum. He was very excited last month when his class read about it and he got to bring in his pictures. If you go here make sure to ask if you need a ticket for the tour. Even though it is free, they want to limit the size of tour on busy days.
Natural History Museum: Very cool place. It was my 7 year olds pick because of the dinosaurs. We all had a good time here. They even had a little kids area at the top where my little one could play for a bit.
Science Center: Literally right next to the Natural History Museum. After visiting NHM, we went over here to check out the space shuttle. It was very cool! You need a timed ticket, which cost $2 per ticket. We went over around 1:00 on the day after Thanksgiving and got tickets for 15 mins later. They have a lot of other cool exhibits, but we ran out of time.
LA Zoo: It was one of the only things open on Thanksgiving, so we went there. It was nice, but nothing special. We are members at our local zoo, so we got 50% off admission prices. With so many other unique places to go, I would put the zoo at the bottom of my list.
I thought about the Griffith Observatory, but figured my then 2 year old was too young and my 7 year old wasn't interested. I also heard a lot of great things about the Farmers Market, but we did not have enough time on the day we did the Tar Pits.