They are such different parks! Really Busch Gdns, whilst it does have many animals, is most appreciated by big ride lovers.
Sea World has a combination of continuous viewing areas and shows such as the Shamu Adventure with the killer whales, and the Sea Lion & Otter show to name only a couple, which are shown several times throughout the day and are very entertaining.
They now have added Journey To Atlantis and Kraken for some thrills - Kraken is highly rated by Coaster lovers and J to A is a great ride if you don't mind getting soaked !!
I think you need to inform the children what each park is like and see how they feel about it.
( If you do pick Sea World, when you pick up your park guide on the way in, you'll see the show schedule listed - be sure to give yourselves PLENTY of time to get to the show location, get seats, relax and enjoy some very good pre-show. The worst thing with this park is to try to rush from one show to another and get there at the last minute).