Ive taken two 7 year old boys to
Legoland now and they love it. I would reccomend going there before
Disneyland though, as it suffers in comparison. It is a very different kind of park, much more geared towards smaller kids. But there is plenty there for older kids as well. What I like about Legoland is that is so much there is kid-powered, and depends on their input to go. The room where you build a
lego car and race it against other kids, the firetruck ride, the Driving School. My boys love legos, so this was something they very much enjoyed, the rides, walking around and looking at the different exhibits and then Miniland.
I agree though, that if you get there when they open and work in a circular and organized pattern, you can be done by 3pm or so. Plenty of time to hit the beautiful beach there in Carlsbad for a few hours and relax, have some dinner and then head back north after rush hour is over.
Do NOT miss the apple fries with vanilla cream dipping sauce at Granny Apple Smith's (or something like that) in the castle area. These things are sooooooooooooo good! Also, if you have lego fans, the factory tour where they show you how they are made was a big hit and then dumps you in the shop where they have bins of every lego known to man to mix and match and purchase by the poound.