Age-appropriate yoga and mindfulness practice including breath work to calm, some asana poses to stretch and move energy. And a tiny bit of stillness (they are kids!) to teach relaxation skills. Younger student classes may have a story or themed class, while older student classes will follow a traditional introductory yoga flow.
Bring your own mat if you want, teacher will have some to borrow if needed. Learning tools to manage stress, anxiety, social pressures, and other life challenges in a healthy was is important to live a joyful life, and starting early will set up a great foundation.