Yoga is proven to help sleep. It also improves confidence, body awareness, food/life choices, self discipline and teaches both gratitude and connection with others. Children who have a spiritual anchor are also less likely to suffer from anxiety or depression; yoga is a spiritual practice. It can help empower children to recognise feelings of anxiety and give them the tools to deal with them. More and more studies are being published to validate the power of yoga. Here is some recent research from UCL on childhood depression
http://www.ucl.ac.uk/ioe/news-events/news-pub/sep-2017/one-in-four-girls-depressed-age-14-study-reveals