This video from Greater Good Science Center offers a simple practice to help foster gratitude in children: 

Four Ways to Foster Gratitude in Children 

Grateful kids and teens are less likely to experience depression or jealousy, and more likely to do well in school, according to research from the American Psychological Association. Researchers have identified four parts of gratitude that help children practice gratefulness using the “notice-think-feel-do” questions: Kids may not always be able to answer all of these questions, but practicing them will reinforce the habit of expressing gratitude and appreciation over time.