I absolutely LOVE SW.
Yes, one day is enough, provided that you get there early, and plan on moving at a pretty constant pace. (Last time I went, I wore my pedometer... walked over 6 miles that day.) If you are a roller coaster fan, hit Kraken first... the line grows as the day goes. It's worth the wait for a front row if the line's not too bad!!! The Journey to Atlantis line grows all day, too.
Voyager's is a great place to eat lunch (in the Waterfront)... woodfired pizza thats better than any pizza I've ever had in a park!!! They have lots of other good "non-fast" food served cafeteria style, so it doesn't take too long, and is worth the real 'meal' as opposed to fast food.
Obviously, you don't want to miss Shamu or Clyde & Seamore. (Just don't sit in the splash zone unless you REALLLY want to get wet.) The penguin encounter is a definite MUST SEE, as well as the dolphins, and the sharks. I am partial to visiting the Clydesdales on every visit(as they are my company's mascot), but if the kids have never seen THAT big of a horse THAT close, they'll be impressed! I think "Pet's Ahoy" is definitely optional... haven't seen Oddyssea yet, so I can't comment on that one.