Onomatopoeia

Onomatopoeia is poetry and a language all on it's own. In Latin and Greek the word means "name making". Poets use this device in which words they use sound like the very thing being named or written about. The sound imitates the actual sound to which it refers, such as "pop", "sizzle" or "crash".
 Most nursery rhymes are composed around the use of onomatopoeia, a form of poetry with sound words as it's focus, children have little difficulty in creating a word that imitates the sound it represents. This perception of sound can be demonstrated through the instruction of simple songs such as nursery rhymes. The rhythm of nursery rhymes have a repetitive pattern that young children can follow and retain long after the song is over.
As well as helping young children grasp the rudiments of language, nursery rhymes have been used as a form in which to pass on the memory of historic events, which most of us have long forgotten. For example Ring a Ring o Roses refers to the Bubonic plague and the accompanying red rash that appeared on the unfortunate victims. Remember Remember , alludes to Guy Fawkes failed attempt to blow up the English Houses of Parliament. Many of the words and meanings of nursery rhymes were used to parody the royal and political events of the day, as direct dissent would often be punishable by death.
So the humble nursery rhyme has a two fold purpose in the basic form of poetry.