Your kids are the perfect age for
LegoLand and it is a fantastic park. My kids were 10, 9 and 6 when we went and they all loved it. The rides are fun, what they do with legos is astonishing, the shows are nice,and the lines aren't long (at least when we went). The best part is that parents like it too! While i would call it more of an amusement park then a theme park, it is well worth a day. We got there when the park opened and left when it closed at five that night. We easily could have stayed another three hours. Remeber that Carlsbad isn't exactly near Anaheim and LA. It's closer to San Diego and SeaWorld.
Also check out the California Science Center in LA. It is great for kids, although yours might be a little young. La Jolla is a beautiful town to walk through if you get down to southern California. The caves on the beach are interesting as is the seal pool, but half a day tops. And bring a lot of money if you go shopping there.
Oh yes, Balboa Park and the San Diego Zoo and various museums in that vicinity are worth a trip.
As a matter of fact, we spent only one day at Disney (I should have never gone to the California Adventure - we all thought it was a waste of valuable time) and the rest in the San Diego area. It's one of the most beautiful places in our great country.
Hope this helps.