We made harps using wooden hangers and nylon fishing line. The idea was posted by another member, however I wanted to re-post here as a lot of head-scratching went into taking that idea, but then making harps that you can actually tune :-). Please see the photos which will show you how we made them. The little washer can be used to tune the harp and as a plectrum to play it.
A word of warning is that they take a while to make, but we have a small church, so it was manageable. We made the harps beforehand as it is too difficult for kids to do, and then during the lesson the kids decorated their harps using permanent markers and stickers, and we also asked them to write "The Lord is my shepherd" on the harp as a reminder that David wrote several Psalms.
The best thing about these harps is that they are playable to some extend, and they won't just go home and be binned. So hopefully it is a lasting reminder of David and his love for the Lord!
Finished "Harp" (4 strings)
Tuning with a washer.