You should take a look at MES English (
www.mes-english.com), which has a lot of useful material and ideas for children's classes. You should also take a look at ESL Teacher Talk (
ESL Teacher Talk), a site with podcasts on teaching. They've had a few talks about lessons for children.
Ultimately, I think you want to choose fun activities that have the kids moving around. Objects, colors, and simple introductions and/or sentences will go over better with their attention spans than any grammar explanations or long sentence structures. Consider how many children will be in the class, too.
Good luck!