We went to Legoland about two years ago with my two sons age 12 and 8. They LOVED it! It is not for everyone though. I agree with the previous poster, that if you like lots of thrill rides, then Disney would be better. BUT, if like us, your kids are not big on the scary rides and LOVE Legos, then you will have a great time! My older son could have sat in the Lego area and just built Legos all day. We went in August and it was not horribly crowded. Don't know about Christmas, but I would think that would be a good time. Southern California is not as warm as what we Texans are used to. Even in August, the ocean was FREEZING! Go to the Legoland website and sign up for email offers. We got some good deals on our tickets. Depending on how much time you have, you might consider a two day ticket for not much more. My kids loved the Lego factory and the Lego store where you could buy individual Legos by the pound. Of course, now they have a store like this at NorthPark, but at the time, we had never seen this. They have a much bigger selection than locally.
Don't let people discourage you from going. We too were told that our kids were too old and would be bored, but we knew our kids. They have no interest in Disney or even Six Flags, but they both loved Legoland and have asked to go again. We were somewhat disappointed in Sea World, since we had been to the one in San Antonio. The San Diego one is not a big and does not have a waterpark.