Kids yoga needs to be fun, light and always changing.  I have found that using a theme throughout the week is helpful when weaving different aspects of learning together.  Often I will begin with a story and throughout the week we will explore the animals, plants and other imaginative aspects of it in our yoga asanas.  If you are planning a three hour session then a short break in the middle with health snacks is a good idea.  Also the kids I have worked with love creating their own unique postures in small groups and acting these out to the larger group at the end of the day.