Project Gutenberg is fantastic.@Barbara, you could try searching Project Gutenberg or other public domain sites for 'nursery rhymes' or something specific - if it's there, you can use it. Here's Andrew Lang's The Nursery Rhyme Book.
nursery rhymes a surely in the same vein as 'folk stories' think of hans christian andersson or the brothers grymn. they sourced their material from well layed down sources... i am pretty sure 'humpty dumpty' harks back to the english civil war...or even 'Ring a ring of rossies' is a refference to the pbubonic plague in London ??? cant see anyone giving you a hard time if you use something that old ?