Half a bag of chips?

My five-year-old came home from school the other day and asked his dad if he could have a small bag of chips. Dad said, "No, because you never eat all your dinner." Child said, "Half a bag?" Dad said, "Ok, half a bag." Dad handed over the bag. Child at the whole thing. Dad said, "You better eat all your dinner!" Child said, "I will." Child did eat all his dinner! He made it clear he was going to eat the entire thing. I think he wanted a cookie afterward, which brings me to another topic.

This same child always asks for some type of dessert after he finished his dinner. Last night it was, "Can I have a cookie now?" UGH! I flat out said, "No." He asked why and I told him it was because he is always expecting dessert. We don't have dessert on a regular basis. I'd rather he have a surprise dessert instead of expecting something every night.