I just got back from CA. We were only in LA briefly so I don't have a ton of recommendations, but we did have a 12yo with us. We stayed at the Magic Castle Hotel--which is not fancy by any stretch but was unbelievably kid friendly (free candy and ice cream, a popsicle phone by the pool). It's near Universal. We were able to go to brunch at the Magic Castle (which is a private club for magician's) which was also really fun.
My 12yo is not really into movie stuff, but we did wander around the walk of stars and Chinese Theatre (walking distance from the hotel) and she liked that. You can look up where the stars of people she likes are.
She absolutely loved Griffith Observatory. I wasn't expecting it, but it was one of her favorite things about the trip. We ended up not going to Santa Monica because of weather, but she had been excited to see the Pacific Ocean.