While you can hit most of the highlights in one busy day, my personal advice to first-time visitors to SeaWorld is to give it two days (or one full and one half day, at least). One of the nicest things about SeaWorld is that you can slow down and relax and enjoy the attractions at a much more leisurely pace than you can at the Disney parks. If you have only one day available to you, then by all means visit SeaWorld. But if you can squeeze in a second day, then that would be my recommendation.
We are big fans of the all day dining. It is truly unlimited.....you can get one entrée, one side or dessert, and one drink each time through the line. If you don't want all 3 (entrée, side/dessert and drink) you can get just one of the three (such as a bottle of water). Or two of the three (like orange juice and a slice of pizza). Choices are awesome (think chicken and rib BBQ platter, fettuccini alfredo, fish and chips, turkey bread bowls, wraps, salads, pizza, sandwiches, etc.), food quality is excellent, and the plan is accepted at most restaurants in the park (Sharks Underwater Grill and Dine with Shamu excluded). It even includes breakfast (served at one location only each day). So if you are planning on arriving at park opening and leaving at park close, eating 3 meals there (breakfast, lunch, dinner) plus unlimited snacks and drinks between meals, then it can turn out to be a GREAT deal. There is an offer out right now for free all-day dining with a full price admission using the code "playanddine" when purchasing tickets on the website. Then I'm pretty sure you can add your second day for $10 by visiting guest services before you leave the park.
Must dos? Well, for kiddos the ages of your children, I would say the One Ocean, Blue Horizons, Clyde & Seamore, and Pets Ahoy shows would all be definite must-sees.
They will probably want to spend lots of time in Shamu's Happy Harbour, where there are an assortment of kid-centered rides. The whole family can enjoy Antarctica, the SkyTower, Turtle Trek, and Wild Arctic together (WA has a mild side if that's an issue). If there are coaster lovers in the family, Manta is incredible and Kraken is also a very good ride. Journey to Atlantis is a water flume ride which will probably appeal as long as your children won't be scared by the one sharp drop.
There's multiple opportunities for animal feedings. I am guessing they will want to do more than one, if they are like most kids

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Then, when you add in the aquariums, the Shark exhibit, and the other little features of the park that you will want to stop and take in (plus the family favourites you might want to do or see a second time)......you can easily fill a day or two!