Favorite parks are such an individual preference. We like all the parks, but for my family and my 14 year old son, Sea World is our very favorite park. This has been my son's very favorite park every year from age 3 when he first visted and still is his favorite park. He'll be starting high school next year. The marine mammal shows are just wonderful, the Manta and Kracken are excellent coasters -- Manta is the first roller coaster I've ever been on my stomach for and is really a blast, the park is just beautiful in terms of landscaping, and has great animal exhibits. For the ones that are still elementary school age -- Shamu's Happy Harbor is an absolutely incredible play area that offers much even for older elementary age kids -- probably the best play area I've ever seen. There is the Wild Artic simulation ride that you take before going into the Wild Artic animal exhibits that all ages will enjoy.
If it were me, I'd just take everyone to Sea World. It offers so many things that diverse age groups can enjoy together too.
The Harry Potter area will probably be open in IOA when you are in the area. This I think is a double edged sword for you and makes the choice of whether to go to IOA or not tough. Many in your group have probably seen advertising and may want to see the new theming. On the other hand, IOA, may be extremely crowded and not as much fun because of this. Will you be going high or low season? That could play into whether I'd want to try this with a large group too.
Don't sweat it out too much, though, in Orlando (the capital of the world in terms of world class theme parks), in my opinion there is really no bad choice. Any park you don't get to is just a great excuse to do another trip another time because they are all excellent.
