Hey bb- Last year I did a type of theater camp for my 4th and 5th graders. My co-worker had some storybooks that were accompanied by a CD. I used the books to make scripts for the students and I also added stage directions to help the students know when and where to go on and off stage. I used the CD's along with the free program audacity [http://audacity.sourceforge.net/] to make sound files for the plays. Throughout the week I had the kids practice their lines and make props for the plays (masks and such for their characaters). It took some time, but it was worth it. The kids had a blast with the lesson.
My suggestion would be to find some storybooks that are accompanied with a CD and use those files to make sound effects and perhaps songs if there are some on the CD. Otherwise, you can look online and find popular kid songs on youtube and use them. If you don't have any storybooks available, then you can go to this site [http://storynory.com/] and find some stories to choose from. You can probabaly find some sound effects on the internet through google. Hope this info is helpful for you. :)