Obviously, kids eat a lot less than adults. And the younger they are, the less they will eat. So, I would say that you should first get the age range of all the kids. If you have a kid roughly around 4-7 years old, they will likely eat a slice each. But older kids around the age of 12-16 will often eat 2 slices. A 12-inch pizza should feed between 4-8 kids, depending on their ages.