My all-time favourite book for children about poetry is Love That Dog by Sharon Creech, but beware it is a little sad and it's directed for children about age 9 or so. It provides an introduction to some wonderful poetry and should inspire a child to explore some poets that they might not "meet' for a while.
Also:
Mother Goose
Dennis Lee
Shel Silverstein
The 20th Century Children's Poetry Collection - Jack Prelutsky
A Family of Poems - Caroline Kennedy
Jabberwocky - Lewis Carroll
Old Possum's Book of Practical Cats - T.S. Eliot
A Child's Garden of Verses - R.L. Stevenson