We did this for our DD's wedding 6 years ago and last May for her Concert and Show Choirs from the high school who were there with her on a competition trip.
Six years ago, the minimum was 40 people and the cost approximately $15 per person.
Last year, we had about 60 people and the costs had gone up considerably - the simplest dessert party "For the Child in You" was about $18 per person. It was fabulous - included Rice Krispy treats, brownies, big cookies, lemonade, etc. They provided a CM to keep everyone away from the group and servers to help the kids. This was on the lower tier of the bridge between France and United Kingdom.
If you want to set something up like this you need to call Disney Catering/Group Dining and they can help you.