Every so often, the topic of "what are some good adult songs for kids" comes up on one of the blogs which touch on children and children's music, including this one. The Beatles are always well represented. Number one by a good margin on my list is "Three Little Birds" by Bob Marley. Here's a nice rendition by Brazilian singer Giberto Gil.
Mr. Gil has led quite an interesting life, including stints playing with Yes, Pink Floyd, and the Incredible String Band in the 70's, and is currently Brazil's Minister of Culture. More about him at wikipedia.