If your child is prone to anxiety or has trouble falling asleep, Kate Holcombe recommends the following mediation practice, appropriate for 6-10-year-olds. Once children become familiar with the practice, she says, "they can do it anywhere-in bed, on their yoga mat, or in the car on the way to school."
Sit comfortably together or lie down in a quiet, relaxing spot. Keep your instructions simple.
First ask your children to place a palm on their belly and begin breathing through their nostrils. Then ask them to feel their belly expand under their hands as they inhale, and feel the belly gently sink as they exhale. Start with 6-12 breaths and continue for up to 5 minutes if they're game. You can prompt them with suggestions like, "Let your mind pay attention to your quiet breathing and the peaceful place you feel inside."
You can also ask more experienced children to begin making the exhalation a bit longer to further activate the parasympathetic nervous system, which triggers the relaxation response.
Source: Yoga Journal Magazine