The way to instill a curiosity or desire in our youth to take up yoga lies in making it fun for them. I share what I love with the the children I teach, and my goal is to make yoga enjoyable for them so that they can stay engaged with their yoga practice to learn fitness, relaxation, concentration, and body awareness.
Have kids try the Swan pose, which stretches the neck, arms, and legs. It is a great chest opener which also strengthens the low back. Not all children can touch their head to their toes in this posture. Everyone's body is different, and the main idea is to do the posture according to your own individual body type. Kids will get 100% benefit by just trying the pose to the best of their own ability.