If you need to dispute why you should not believe in Creationism, see 15 Answers to Creationist Nonsense from Scientific American.
Understand flaws in argument yourself, encourage children to practice good logical argument and be able to identify flawed argument. Start from Fallacy from wikipedia and follow some links for further information.
If you *really* need to teach Religious Studies [yes, I see the cultural importance of understanding the things that have been done in the name of "God"], please do the following:
1. Teach comparative religion. Make sure you invite at least 3 different religious leaders to speak to your class from 3 different religions. If you can, please invite a Buddhist monk.
2. Give this website as a resource when you study Bible: Skeptic's Annotated Bible / Quran / Book of Mormon
3. Be honest with history. Discuss how religion was used for political reasons when convenient and how new religion was created when it is inconvenient.
4. Most important or all, DO NOT VIOLATE the children and force them to believe your belief. [If you really believe in God, at least give your children a choice so that they can choose their own. Frankly, there are so many versions (that's why there are so many different religions each claiming theirs is the true God. Yet there is no overwhelming evidence to show who is correct!). Please at least acknowledge that YOU may have chosen the "wrong" God to believe in.]
5. Be HONEST. Don't lie to your children. Present facts within statistical evidence (i.e. demonstrate whether an occurrence is statistically significant or not). One person cured out of 1 million IS not a miracle!