Being around people who pronounce the words correctly or poetically is important to young children. If a young child is around people who use a lot of slang he will learn a lot of slang. If he is around people who use only standard English, he will learn that. If he is around technical jargon he will pick up some of that. Around poets he will pick up many conventions of the form and art.
Children are never too young to hear poetry, or too young to write it.
Help them make books of their words and pictures. If you can get a child to draw several pictures you can then ask about each of the drawings and write the descriptions or comments. Make a book of the pictures and words that go with them. Here is a poem recited to me by my daughter at about age 4 and a half. She made up the poem, and I wrote it down for her and read it back to her.