The longer I teach yoga to kids, the more I look for fun ways for children to experiment and enjoy moving their bodies. Creative movement is such a wonderful way to make exercise and stretching fun! Here are some of my favorite creative movement activities (and although they aren't all exactly yoga poses, they are mind/body exercises):

Dance ribbons- cut long pieces of ribbon or tulle, and tie them onto the end of a stick, unsharpened pencil, or even chopsticks. Turn on music and twirl. You can lead the kids in poses too- arm circles, painting rainbows, etc.
Egg Shakers- one of my favorite props ever... Use Laurie Berkner's I Know A Chicken song.
Hokey Pokey- a classic, but uses balancing skills that help with yoga poses
Head, Shoulders, Knees, and Toes- Touching toes is a forward bend.
Yoga Flows- Let the kids make up their own flows using yoga poses or their own movements

A find that "creative" time is a good balance to poses that require some attention to alignment (Like the warrior poses and Triangle). Reward attention paid to alignment with some creative time afterwards... Or, use creative time to release some energy before moving into poses that require some stillness.

Have fun!