For young children, I think that rhythm is very important, as is poetry that tells a story and poetry with strong imagery that is not too metaphorical. Children are drawn naturally to strong rhythm, which is like music to their ears. It helps them to recite and remember, and it even seems to encourage them to move about in the rhythm, so they are feeling it in their bodies. Children also seem to like a "plot" of some sort, rather than poems that are a snapshot of feelings or concepts. Few young children can understand such poetry yet, and they need to be captivated by a story. While children often understand some idioms (See Amelia Bedelia), I would be careful with metaphors and similes in introducing poetry to the very young. They usually do not have enough experience to understand, for example, that a tree can represent the death of a friendship, perhaps not until eleven or even twelve. When I think about my own childhood and what was the most successful for my children, I immediately think first of nursery rhymes, which are strongly rhythmic, tell a story, and are not overly-reliant on sophisticated metaphors. In stepping it up a bit, I moved on to poems such as "I Know an Old Lady," some simple Robert Frost, and Walter de la Mare. I also remember my father reading me some fairly lengthy narrative poems, such as "The Rime of the Ancient Mariner" and being captivated by them. I would also say that if you are reading any poetry to children, that is wonderful, no matter what you are reading.
The elements of poetry include imagery, voice, plot, diction, stanzas, rhythm, sound, meter, form, rhyme, metaphor, and repetition of words or other elements.
It's essential, I believe, for young children to be exposed to quality poetry and eventually get to learn and understand the elements of poetry. This will give them a greater appreciation of the art form. It will also give them greater appreciation of the work that goes into creating good literary poetry, whether formal or free verse poetry.
For instance, learning about stanzas will teach young children that a stanza is a way to group similar thoughts on a particular idea. One stanza can focus on one idea concerning a theme, another stanza can focus on another idea concerning the poem's theme. Children can also learn that stanzas can be of varying lengths and learn how to structure a stanza to fit the poem they are writing.
In addition, young children can learn about the elements of free verse - poems that don't have a customary model or pattern, as opposed to formal poetry such as iambic pentameter, which does. Blank Verse has a set pattern though, but is unrhymed, but is still considered formal poetry.
In conclusion, knowing the elements of poetry will assist young children in creating elegant poetry that can convey a good message on a topic to the poem's readers.
Thank you Lorraine and Michael for your answers, they are so helpful.