Jennifer,
I used to help out with Kindermusik classes. This sounds kind of similar. There are some short stories out there–I seem to remember something about a “Harp Mouse” that one of my teachers used with her younger students.
You might even want to try teaching them something they know–like “Jesus Loves Me” or “Twinkle Twinkle Little Star”. With something radically different like the harp, it helps to give them something they know to base everything off of. Otherwise they just might forget it.
Also, kids love texture and color. That is something that was really big in the Kindermusik. If you have another instrument available to you (guitar, recorder) you can show them and let them feel the comparison.
I’d go on a bit about the string colors, too. People always seem to find that fascinating. Let them count with the strings. Teach them the “music alphabet”.
Thought I’d throw all that out there. Good luck–have fun–take pictures! You’ll want to keep some of those expressions forever!
~Fairy