Here's a list of healthy treats for chickens:
It's important to keep in mind that treats should not be more than 10% of their daily diet. You want the birds to eat their nutritionally balanced feed and not fill up on goodies...just like people, they aren't going to grow and function at their best if they fill up on snack food. And if they gain too much weight from overeating, that will interfere with laying. I wouldn't worry too much about 1 incident of pumpkin pie but wouldn't make a habit of it.