They typically let you in at about 3:00 pm. There will be a ton of screaming kids there, so be prepared for that. Your older child will want to get slimed, so make sure you get him out to the lagoon pool every day before 3:30 so he can get picked to play the game show out there. If he doesn't get picked, he can get underneath the big bucket at the lagoon pool at 4:45 and the fill it with slime and dump it. Have you made reservations to the character breakfast? You should, it was a load of fun. You can make ressies when you get there. There is usually an assortment of characters there, these are the possibilities: Spongebob, Patrick, Dora, Diego, Cosmo and Wanda, Tommy Chuckie and Angelica, and Blue. Characters also appear in the Mall at random times, so check for that too. If you want, you should try to get tickets to the Studio Nick shows (Specifically Nickelodeon Live), they are free and very fun. If you want, there is parties that you can go to, but they cost some $$. If you decide to go to one, Try to go to the Slime one or the Spongebob one. They will be just the right amount of fun and messyness for your son.
Have a great trip! (Say Hi to Spongebob for me!)