In this busy world our minds become cluttered with so many thoughts, feelings, and chatter.  I believe that children and families that meditate together can find a safe quiet place to refocus and clear out all the cobb webs and dead bugs.  This practice when learned and used daily can help with our mental health and physical health.  My daughter has Autism and Epilepsy.  She can get agitated, weepy, angry or sensory seeking/avoiding.  We use Yoga as a way to help her calm and clear her mind.  We use Silly Yoga to get out all that extra energy so that she can relax or focus.   For myself, I have been a dancer for 45 years , I had an accident that limits my dancing but I with Yoga I have been able to recover mostly.   I enjoy Yoga because it takes me to that special place I used to go when I danced, that place inside of me that was calm and confident.  
Parental Guidance
Please feel free to join in with your child or not.  Ask them questions like:  what is your OMwork? What is your favorite Yoga pose. Can you show me your favorite pose? Help them find a quiet place to practice yoga and mediation.  Eventually, they will get really good at mediation and will be able to focus and clear their minds in very noisy places but for now they will need to have a quiet space for the class and to practice.