I'm mom to a three year old, a four year old, and a nine year old.
After consulting with the nine year old, this was our conclusion:
Sea World is a must!!! I think its going to be the subject of my next guidebook since my nine year old loves it so much. It has fast roller coasters and fascinating aquarium areas as well as exciting shows. If you purchase a fun card, you can return every day during your visit as long as it does not include the black out days. Check out the website for more info. If you decide to go to the zoo or wild animal park as well, there is a combo ticket available at Costco's website
Legoland is a hard one. I love it so much I wrote a book about it, that being said eight is the upper limits of fun for that park if you don't have little brothers and sisters in tow (some how, little ones give your older kid permission to try things that aren't considered "cool". If your kids love Legos, it might be worth a visit. If you are mostly looking for rollercoasters, it might not be the place for your family. There are two exciting roller coaster rides, a few more rides that the kids will enjoy like the Driving School, a very fun water play area, and fun interactive activities like Build and Test and
Lego Mindstorms (sign up early as soon as you enter the park). The Hideaways is a fun climbing play area that keeps the kids busy for quite a while. If you are visiting in September and your kids are big Star Wars fans, Lego Star Wars day is really fun. Mini-land is really amazing for Lego fans. I wouldn't recommend the Sea Life aquarium for eight year olds. The Birch Aquarium is also small, but has cheaper admission and is more oriented toward older kids. I'm not familiar with hotels by any of the other attractions, but if you like nice hotels, the Grand Pacific Palisades and The Sheraton in Carlsbad are Legoland Resort Hotels and designed to be kid friendly. (Justin had fun playing Wii at the Grand PPP kids club). Carlsbad beaches are really nice, mostly free parking, great places to boogie board. Life guards are on duty during the summer. We also have a skate park if you kids like to skate and bring their boards and safety gear. Discount tickets are available at Costco and include minature golf.
Hope this helps! San Diego is a wonderful destination for family travel. I know you'll have fun.
-Bridget Smith
Author, The Unauthorized Guide to Legoland California
Family Adventure Guidebooks