Stolen Cookies

Riddle: Sheila baked a batch of delicious peanut butter cookies for dessert later in the day. She couldn't eat them while near her husband, though; he was severely allergic to all types of nuts. Plus, the couple's three kids–David, Amanda, and Frank–were all grounded and not allowed to eat sweets for a week. When Sheila went to get the cookies later in the day, she noticed that all of them were gone. She knew it must have been one of her kids who ate the cookies, so she interrogated them. David said that he was helping Dad to bake an apple pie in the kitchen. Amanda said that earlier in the day, she and Dad were eating peanut butter and jelly sandwiches for lunch. She didn't have time for cookies. Frank said that he was doing his homework upstairs. Sheila instantly knew who was lying, and grounded that child for another week. Who was the cookie thief?