From what I can tell the $100 gift card is to help purchase sets for the participants. My concern is a bunch of kids wanting "stuff" and how to control it. I guess the lego store by us will "put a few choices (made by you) on the checkout counter and the kids are told they can pick from there". You pay for any "overage" due to more expensive sets or tax.
I think you can also just take the $100 gift card home or use it to buy goody bag items (like minifigures, small sets) and not have a choice in-store. The sales person also said you could use it to buy basic sets and have the kids build them there (but I would be worried about missing pieces, keeping it all together etc).
Our store, you cannot have any food. You could go to the food court and do food/cake.
The package is for 10 kids. I think you can add on more kids by paying per child (not sure how much or how many kids max).