SeaWorld is always 2nd day free with any 1 day ticket. It is certainly at least a 2 day park to get the best experience. Note that parking is not free the 2nd day, and is currently $14 per car. Get your 1 day ticket vaildated at CS booth. 2nd day must be used within a week of 1st.
With kids 5 and under, Shamu's Happy Habor can easily swallow a half day or longer. The slides, nets, sand areas alone keep my kid in heaven for a LONG time. A kiddie coaster and several kiddie rides top off the area. Also for the younger ones, don't miss the seasonal Elmo show. It is in the Pets Ahoy Theatre, which is also a great show on its own for little ones. Sit near th front at the Pets show. As soon as they are done head to the stage. The pets will be brought up to pet (and the doggies often give out some sloppy kisses to little ones!)
Look for Cookie, Elmo, Zoe, and others to be out posing for photos in front of the Pets Ahoy Theatre from time to time. Also, Elmo usually poses off to the left hand side of the theatre (outside of the theatre) about 5 minutes after the Elmo show is done.
Little ones also love feeding the dolphins (extra charge), petting the sting rays, petting the horses back at the stables, the new aquarium (which has a cool photo op feature where kids can crawl under a tank and pop their head up, making it look like they are IN the tank with the fish), etc.
My little guy loves the penguin area, as well.
Oh, so much more. Really, plan on at least 2 days. After Magic King, it is my 2nd fav park in Florida.