Parents, caregivers, and educators often use food as rewards for children. When parents tell their child they can have cake after finishing their dinner, when caregivers reward potty training children's use of the toilet with chocolate chips, and when teachers hand out cans of soda for good behavior, they are using food as a reward.
From my experience, many parents have an intuition that using food as a reward is probably not the best idea. I think they are right. If it is done consistently and frequently enough, I think some children are vulnerable to associating eating with rewarding themselves for hard tasks. It also takes away from sweet food's intrinsic value and place in regular eating.
I think that this is an area that there is no one hard and fast rule. I don't think food as reward is always a horrible idea that will ruin a child's eating habits for life. It depends on the child, the food used, the frequency, and the messaging given by the adult along with the food.
A chocolate chip as a reward for a three year old cleaning up the toys is not the same as a bottle of soda for a ten year old behaving in school. A rare bottle of soda as a reward is not the same as a daily bottle of soda as a reward. Also, different children will react differently to the food. Some three year old children will be thrilled by the prospect of a chocolate chip, and afterwards they will move on. Other children may start manipulating their behavior to obtain another food reward--even offering to put away the toys because they want a chocolate chip. Still for other children, it turns into a cycle of tantrums that only end when food is given.
The messaging parents and other adults give along with the food also matters. If children get the impression that food is meant to be a reward for good behavior, hard work, or eating nutritious food, it can lead to disordered beliefs about food and eating. Food is meant to be an every day thing and not something we have to earn.
As a parent or caregiver, you need to make a thought through decision about if and how you choose to use food as a reward. Think about the child's personality and the food you are using. I don't think occasionally using small amounts of food as a reward will harm most children. I just don't think it is best practice.