FloraG,
Your problem with children listening and understanding could be because of a multiplicity of reasons. Consider a song intended for adults with some difficult ideas. Why would children be interested? With an audience like this I would explain about the song first to give them a chance. There is also the factor that you are competing with lots of other genres/media for their attention. They are often not used to listening to the words unless you specifically give them incentive or tell them that's what you'd like.

A group I am in recently went into a school to sing to 10/11 year olds. They were using our songs for a heritage project. We started to sing one song I had written for our last album. Imagine my surprise and joy when about 90 kids started to join in not just the chorus but the whole song. We just sat down and played the music while they sang it. Blown away! The song has 8 stanzas by the way and it has a narrative.