Pamela,
I'm surprised no-one has mentioned keeping things quite simple to start with considering your level of audience. There are lots of simple universal tunes amongst the nursery repertoire. One of the simplest 'Baa Baa Black Sheep/Twinkle Twinkle Little Star et al' could be a good starting point. There are probably still people about who haven't even realised these tunes are really the same.

'Ach Du Liebe Augustin' is a simple universally used tune, 'So Early in the Morning'. There are lots of examples.

having introduced these you could then move on to something like the ubiquitous 'Villikins and Dinah' tune before venturing into 'Dives and Lazarus' and on the last one Vaughan Williams put together a brilliant piece using 6 different versions of the tune I believe.