What do you sing to them now? It takes a fair amount of quick thinking, but my own parents used to listen to music and morph the songs to fit the kids. It was funny, and silly, and made a lasting impression, because we still sing some of the made-up verses to each other. I think it would encourage the kids to think, and certainly challenge them a lot more than the majority of the Raffi-esque kid songs out there that say I love you.