It is located in epcot in The Land Pavilion lower floor next to the line for living with the Land ride and is presented by Nestles. Here is the info from the DIS
For kids ages 3 - 10, the Junior Chef Program is a lot of fun. Kids are divided into teams where they make cookie dough from scratch and bake it in Chef Mickey's Magic Oven. Once they're done, the Junior Chefs get to sample their culinary creations. There are Five 25 minute sessions daily at 12:30, 1:30, 2:30, 3:30 & 4:30pm. I's first come, first served and only 20 children can particpate at a time. You'll want to get there early to insure that your child gets a spot