How do your children feel about whales, dolphins, sea lions, manatees, and fish? If they are excited about sea animals, then they will love Sea World. If your 12 year old likes roller coasters, Kraken is a bigger rush than anything I've been on in WDW, and they also have a big flume ride. I've heard the Wild Arctic simulator ride is excellent, too, but we bypassed it because our children are too young. The shows are all excellent. (Don't miss Pets Ahoy!) Shamu's Happy Harbor play area is huge and the kids love it
If, however, they shrug their shoulders at the idea, it could be a waste of time and money. It really depends on what they like.
Regarding MGM, read through descriptions of all the shows to see how many would appeal to your children. Our DS will not go into any of the theaters, so MGM is basically a huge waste of time for us.
We had Fun Cards (annual passes) for Sea World last year, and have FL resident seasonal passes for WDW this year. While our 3.5 yo DS loves WDW, he still asks about SeaWorld and Shamu all the time.
Good luck with your decision and have a great trip!