If you had one day to go to either the Zoo or Sea world with a 9 and 7 yr old which would you recommend? TIA
I have been to both, and my DD enjoyed both when we went. Both are great, in different ways. I would make the decision based on what similar experiences your kids have had, and whether they can be entertained for a whole day looking at animals without shows or rides.
If there's not a good aquarium near you, Sea World is great. The smaller aquariums are pretty darn decent, and the bigger exhibits are all great. the shows are enjoyable, too. There are also a few thrill rides, and some not-so thrilling rides that are Sesame Street themed. You might want to check the height requirements out to make sure your kids won't be disappointed if they aren't tall enough. If you have a good aquarium near you (we do), the smaller exhibits, while well done, might not be super-exciting. It's pretty hard to not be impressed by Shamu, though, ;-)
If there's not a great zoo near you, the San Diego Zoo is amazing. It has many animals that are not at many other zoos in the country (pandas, koalas...), and most of the exhibits are great and fairly "natural" seeming. There's a bus tour you can take, which is nice, and there are a few shows. The zoo is HUGE, so prepare to not see everything in one day. I would recommend looking online at the exhibits so you can make sure to see the animals you're most interested in. We enjoyed our time there, for sure. If your kids aren't going to have a chance to see a panda or koala anywhere else, and they love animals, it's a great choice.
You didn't mention it, but my family LOVED the Safari Park (it used to be the Wild Animal Park). It's in Escondido, which is outside the city, but it's a chance to see many animals in expansive, natural enclosures. If you live in an area with a good zoo and aquarium, it's just a very different experience. We loved the tram safari (included with admission) and saw a newborn antelope stand up for the first time. It's another great choice.
Honestly, I think it's hard to go wrong with any of those choices. You just need to know if your kids can happily spend a whole day at a zoo, or if they will need (or want) more varied entertainment. We would happily go back to any of the three, and would recommend them all.