Onomatopoeia, according to the Oxford Dictionary, is the 'formation of a word which describes its sound'. Examples of onomatopoeic words include sizzle, clap, moo, roar, etc.
It is a common feature in many poems written with children in mind. The onomatopoeia can sometimes form a refrain, that repeats through the poem, providing structure.
The following poems on the poetryline website contain examples of onomatopoeia: