Chewing a cookie is physical change. It's a physical change, because the cookie is now in smaller pieces, but it did not change into a new substance. It's appearance changed, but not the cookie itself.
Chewing the food is a physical change, but once the food mixes with saliva it becomes a chemical change. Digesting it is definitely a chemical change, because the stomache acids mix with the food and viola a chemical change.
Chewing food is a physical change but is necessary to release chemical energy from the food. Chewing food helps the chemical process go to completion, sort of like crushing a substance before recrystallizing.