First, congrats! I really loved the place and you've made a great choice.
Second, shuttle service is available, but not that great. I suggest having a rental car or something like that. Also, Set aside a day at the hotel itself and let the kids have a day in the waterpark. Once they see the place, they'll never want to leave.
The food court has a nice variety of options, plus the two buffets that change selections every night. The character breakfast is really good, and you can see Spongebob and Dora no matter what, along with others like Cosmo, Wanda, Tommy, Angelica, Diego, and Blue. The "Mall" area also has a photo studio, a kids spa, a 3-D theater attraction, and two shops (A convenience store and a Nick shop). At 4:45, the bucket on top of the lagoon pool fills with slime and dumps onto the crowd below, so don't miss that, and there are daily Nickelodeon game shows in the mall and poolside shows as well.
The rooms are very nice, IMO, and they all have TVs and Bunk Beds or two Twin Beds. There is also a Playstation 2 in the kids room, so if you have games, bring them. You can also rent them from the massive Arcade. Request rooms nearest the Lagoon pool, it's the largest waterpark area in the hotel. Spongebob rooms cost more than a regular room, but generic rooms have Fairly OddParents, Danny Phantom, Jimmy Neutron, All Grown Up, and I believe a few Nick Jr. Shows as well, so request a specific one if you want.
I personally think it's only worth the money if you aren't going to the parks for at least a day or two while you're there. 2 nights is just right to see everything but not enough to get bored.
OP, I'm thinking your 16 year old will think the place is "kiddie", but it's honestly really cool. Besides, no one will know if he goes to Dora and gives her a hug! I also suggest getting him into the games at the pool. A sliming could be a pretty cool memory!
2nd poster, your 9-year old will likely have a great time, but the 5-year old might feel a bit overwhelmed with the loudness and the crazyness.
Have fun!