The USS Midway in San Diego is an old aircraft carrier that is now a museum. It's really cool and the kid would probably like it a lot. You can do it as a day trip from Ladera Ranch.
The California Science Center near downtown LA has one of the old space shuttles, so that's a cool museum too.
The Reagan Library is in Simi Valley, just a few minutes away from Thousand Oaks.They have an old Air Force One plane.
I don't have older kids, so I'm trying to think of things they might like to see... I know those are all kind of similar, in a way.
If you've never seen any of the old missions, that can be a lot fun, too. San Juan Capistrano is close to Ladera Ranch; Ventura and San Fernando aren't that far from Thousand Oaks.
The Griffith Observatory near Hollywood and the Getty Center Museum in West LA are both really nice. There are some great hikes in the mountains near Santa Monica/West LA/Hollywood.
Not sure when you're coming, but there is a Strawberry Festival in Oxnard (near Thousand Oaks) every May. Or, the Flower Fields in Carlsbad (maybe 30 minutes south of Ladera Ranch) are open March 1 - May 14 and they are really nice. They're right next to an outlet mall. They are also right next to Legoland but, honestly, I think your kids are too old to enjoy it. I don't know many adults that like it very much at all, but the little kids all love it.
Huntington Library (gardens) and Santa Anita racetrack near Pasadena are both great places to visit, too.