We spent a full day and a half at Seaworld, and still didn't take in every attraction or visit every area of the park (no Shamu's Happy Harbour for our 16 y.o., for instance.....and several rides, including Journey to Atlantis and Wild Arctic, were closed). EASILY a full-day park, as the shows are a do-not-miss and you should get to each one early if you want good seating. Quick Queue (front-of-the-line access) will help you squeeze in as much as you can in a single day and maximize your time, so you might want to budget for that if your family enjoys rides.
Unless you are visiting during peak crowd times, Universal Studios can be comfortably be done in a day, especially if you have Express Passes. Islands of Adventure would take longer than US. If doing both parks in a day, pick your must-do's and stick to them, knowing before you go that you'll only be hitting the highlights of each.
If you are planning on visiting IOA anyway, do at least squeeze in a short visit the Wizarding World of Harry Potter. Even if you choose not to wait in the queues to ride Forbidden Journey, the whole area is so well themed it would be a shame to miss if you were at that park for other attractions. Go for a butterbeer and to take a few pictures of your family in Hogsmeade. I am no Harry Potter fan, yet was very much impressed by the theming in this section of the park.