The SeaWorld Halloween Spooktacular is a family-friendly event that starts at noon and is included with your regular park admission ticket. Dress the kids up in their favorite costumes, equip them with a trick or treat bag (or pick one up at the park), and hit the trick or treat trail.
Candy is just one part of the overall celebration. You can see scenes from this annual event by checking out the list in this article. It's great for youngsters who want more treats than tricks, and even adults will smile at the sight of the little goblins enjoying themselves.
I recommend arriving at opening time and using the morning to see as many shows and ride as many rides as possible. That way, you'll get a lot done before the holiday festivities kick off at noon.