Our 'Calm Kids' sessions include beginner meditations for various age groups up to 18 years old. The Calm Kids meditations can found on our website here: https://www.calm.com/program/Ly18ZPL/calm-kids. You can also find these meditations in the Calm mobile app, under the 'Kids' category of the Meditate screen.
The full library of content in the app is also appropriate for all ages. Any of the meditations on the Meditate screen that are not a Series (such as the Breaking Habits Series) or a 7-day or 21-day program will have session options as short as 2-3 minutes, up to 30 minutes in length, so you can select the time length best suited for your classroom.