Jealous Ninja: a Social, Emotional Children's Book about Helping Kids Cope with the Green-eyed Monster - Jealousy and Envy. Do you have a child who is experiencing envy or jealousy? "It's not fair!" is an often used phrase when children are feeling jealous. Whether it's social or sibling jealousy, this natural emotion can cause anger, anxiety, and even sadness. Jealous Ninja learns how to change a SMALL mindset into a BIG mindset.