A couple months ago, my kids learned a song called "Grown-ups Come Back" on Daniel Tiger's Neighborhood (same people who did Mr. Rogers) from Netflix or Hulu. They sing it all the time, and it helps them when I have to leave them.
Watch a clip here: http://www.pbs.org/parents/daniel/?guid=96c57640-7bf3-413a-8afa-a6eb530989d1
Be careful! It might get stuck in your head!