Kids can practice yoga, but from 6-10 practice with mother, some simple asana, and after 10 can practice some more asana, at this age don't let them do too much stretching or flexibility, that  can lead towards the deformities of bones. Practice only under qualified teacher..... If kids can practice Yoga then they can reduce their study pressure, and be calm but can enjoy the study so much, i did in the same way.......