Good for you taking on this important role! I would Google the term "sermons for kids" and look at some of the sites that come up. I've been involved with the children's program at our church, and we had actual materials/books that helped us in guiding the discussion (pretty structured, for ages 4-7). A lot will depend on what is expected within your parish, what resources you have available, how many kids you'll be working with, and how much time you'll have with the children. Has someone else been filling this role already? If so, talk to them for ideas. In general I would try to incorporate some physical "props" to keep the kids focused - they could be posters, toys, household items - whatever you can somehow tie to the theme you are discussing. They are not really for the kids to play with, just to keep them engaged in what you are saying. Many of the sites you'll find can offer some suggestions relating to various readings and/or themes. I also like to do some kind of question and answer thing to keep the kids engaged if the number of kids and ages/maturity allow it. They also typically love to learn songs, and there are many religious songs that would work with kids (at least the chorus). Try to have fun with it. I was often amazed at the responses some of these children had they showed how much they really did "get" what we were talking about. (Of course you also typically get some completely unrelated comments, but you just move past them.) Good luck to you!