We did a slow vinyasa flow each morning, plus a few other poses. Here are the things that worked:
1. Keep it short. Aim for 10-15 minutes, but let the children's attention determine the length of practice.
2. Stand in a circle, so they can see each other and have fun.
3. Describe the poses and the transitions between them, with as much detail as possible. Be ready to stop and assist when ever they ask. Encourage them to implement your yoga "suggestions" in what ever way they interpret them...as long as it's safe!
4. Liken the poses and movements to animals, or other creatures. Be ready to explain or laugh at the names of poses! Acknowledge and celebrate the names and comparisons they come up with.
5. Let the children remind you of the joy of being present, fully engaged. Be amazed along with them. One little granddaughter was amazed when she tried clasping her hands together behind her, with the opposite thumb from what she usually uses, on top. "I didn't even know you could do that!"
Lots more great ideas in the BodhiKids portion of Bodhi Teacher Training.