Quite a few balance exercises are described and shown in “Children and Sports Training” by Józef Drabik (
http://www.stadion.com/children.html ). Among them you will find some that are specific for judo or any kind of stand-up grappling. Nevertheless, have your students do them all, even those exercises that seem unrelated to judo. Doing this will benefit your students sense of balance and other coordination abilities more than doing only judo-specific exercises.