Your community must believe you to be competent and they really do want you to be. When teaching a song, you may discover that you have less time than you though you did or that the group isn't catching on as fast as you had hoped. It's perfectly alright to teach just a first verse or even a first line. Please, though, do remember to come back and teach the balance later. Look for music that enhances the Jewish teachable moment.
Different ages require different repertoire.
Older campers will have vocal abilities and musical experience often requiring more complicated melodies and longer songs. However-don't assume this from day one. Even the most veteran members of the group will want the security of a few old favorites and simple tunes. This will give you the chance to learn what they already know, and help you refine your curriculum.