On
Legoland for a 9 y.o., no problem at all! I have 9 and 5 y.o. ds's, and we've had annual passes for a couple of years now; both boys still love it. Be sure to find the places where kids get to build with Legos, as that's especially popular with a lot of the slightly older ones. Also, if you aren't
Lego club members (check the website...I think you can join for free), it might be worth signing up, as they sometimes have discounts that are better than AAA.
I'd do two hotels for Anaheim and San Diego. I've driven that route a lot, and it can be an absolute nightmare. I will admit to being surprised by the person who said they'd not had problems in the summer, because the beach traffic most of the way can be horrendous; however, sometimes you do get lucky. The carpool lanes can help, but only to a certain degree. But having to face the possibility of 20 mph bumper to bumper at the end of a long day is a good way to wreck a vacation...
We are big Sea World fans; I think it is very do-able in a day, though perhaps a long day. It is not nearly as big a place as D'land (and Legoland also is much smaller, or at least they both feel that way), so simply the act of getting from one place to another isn't so exhausting. Also, because of the several shows, you spend time sitting and thus having enforced rest breaks. San Diego Zoo is fun but weigh the cost against your children's interest in zoos; mine do well for a few hours and then want to go. Wild Animal Park can be very hot (though dry) in summer, and is inland and north a bit from SD, so you'd be adding more driving time. Lots to do and see in SD; don't try to cover it all, but pick a few places and enjoy them!