If your kids like animals they should have a lovely day out. Think IOA is better for teenagers (although there are kiddies rides). We went to all 3 on last trip in December & I think the atmosphere at SeaWorld is better, the shows are funny for kids & there is a play area (you can get wet) for them to run around.
I'm sure everyone will have different ideas though.
I'd recommend Seaworld too for children that age.
Too much will be off limits due to height restrictions at IOA and as Mrs D said it's aimed at older kids. Its only the Dr Seuss area at IOA is suitable for that age group and it's a small percentage of the park.
Remember to take swimming costumes or a change of clothes as they will enjoy cooling off (getting soaked) in the play area at Seaworld if you go.
Go feed the dolphins early as they are most active and the crowds less first thing.
I agree, Sea World for young children. IOA is fun, but the rides are geared more for older kids. Sea World has a great playarea for little ones. Be sure to take their bathing suits and towels. There is a GREAT splash area (big boat) or them.
Another vote for Sea World! I love Universal and IOA, but there is definitely limitations on what they would be able to enjoy due to height requirements.