We were just down there a couple of weeks ago - we had 2 boys, age7, a boy10 and a boy11 with us, and all had a great time!
It takes a little under/over an hour from DL to get to Carlsbad, and traffic wasn't bad at all - it was on a Sunday. We also drove to San Diego on a Thursday, and traffic was awful!
There are things in
Legoland for all children's ages, so I don't think 6 and 9 are too old - there are several roller coasters both both little kids and older kids. My boys' favorite is the Bionicle ride, which is like the Teacups! They also loved the Hideaway, which is a giant wooden play structure in Knight's Kingdom. My favorite part of the park was the boat ride, and walking around, Miniland USA - it's amazing!
The main store at Legoland is great, and shipping prices are VERY reasonable! Saved us from carrying boxes home!
There are also lots of great photo opportunities around the park.
I think that the park is expensive, but it is fun to see at least once. We have been twice in the last couple of years, and probably won't go back for a couple more!
We also went to San Diego Zoo last year (along with the first trip to Legoland!). I absolutely love that Zoo, but my boys were bored stiff! After the bus ride around the park, they were ready to leave! It is a huge zoo, with lots of hills, and the little one got REALLY tired quickly! We do go to the zoo in Seattle quite often, so it may have just been *more of the same* to them! It is, by far, though, one of the best Zoos in the world!
Regarding hotels: you might check out Expedia.com, and do a hotel search for Carlsbad - there should be a whole list of places, along with reader's reviews, photos, prices, etc.