YogaEd has great yoga classes on Youtube for children and teens!
Calm is an app that can be downloaded on any smart device. It provides access to mindfulness sessions for kids and adults.
GoNoodle has many different mindfulness and movement videos for kids! It can be accessed online or downloaded as a channel on some smart devices.
Cosmic Kids Yoga has a ton of videos which blend yoga and story telling. It can be accessed online and there is also an app that can be found on most smart devices.
Mindful Practice's Class Catalyst offers some helpful 'Mindful Minute' videos on their Youtube channel.