Sea World is so much more than just "fish" (whales & dolphins are mammals by the way - ha,ha!) We absolutely love Sea World, and so does our 19 month old baby. We love anything to do with marine life, and so it's an awesome park for us because it's not only educational, but fun as well. Plus, I am obsessed with penguins, and this is probably the only place where I can touch my fav animal! Based on the ages of your kids, they would probably really enjoy Sea World a lot - especially the dolphin & whale interaction.
That being said, we aren't huge ride people, so if you compare it to Disney or Universal for rides as I hear many people do, then you may be disappointed, but if you look at it as a place where you can interact with marine animals, then it's a great value in our opinion. I guess you really need to prioritize what your family likes to do, and then you can tailor your expectations to fit this if possible - more specifically, for us this park exceeds our expectations because our priority is interaction with the animals at Sea World, and not fancy rides or attractions - we visit Disney or Universal for those. Seeing as how your kids are younger, they are probably going to thoroughly enjoy seeing dolphins, penguins and whales for example more than going on a ride that lasts a few minutes or a few seconds in some instances!
Good luck with your decision, Tiger