I like poems which tell a story, particularly for children, Something like "The Midnight Ride of Paul Revere" is a great way to capture a child's interest in poetry form early on. It's also very rhythmical and easy to memorize, even. What an easy way to enjoy both history and poetry!
"maggie and milly and molly and may"
maggie and milly and molly and may/went down to the beach (to play one day)
and maggie discovered a shell that sang/so sweetly she couldn't remember her troubles, and
milly befriended a stranded star/whose rays five languid fingers were;
and molly was chased by a horrible thing/which raced sidewayss hile blowing bubbles; and
may came home with a smooth round stone/as small as a world and as large as alone.
For whatever we lose (like a you or a me)/it's always ourselves we find in the sea.
-e. e. cummings
The sound effects of this poem delight children. The alliteration, the rhyme, the lack of capitalization all move the poem at a quick pace. Since many children in our state have been to the white, sandy beaches, [before BP destroyed them], they quickly relate to the subject matter. The more thoughtful do catch the last line's significance, too, although some are not exactly sure about what it means.
I'm not sure what age children you mean here, but Lewis Carroll's "The Jaberwocky" is a great poem for elementary school. First of all it is imaginative and fun. It has a pretty standard rhyme scheme and stanza structure, which is actually easier for children to comprehend in poetry than free verse. But it is also a cool way to explore vocabulary in context. There are so many made up words in it that children's imaginations are the only dictionary. This is one that be discussed on many levels (what is the Jabberwocky? What is the speaker really afraid of? What are the 'jabberwockies' in your life? Etc.) Then, when you are finished, you can have students attempt to write their own poetry using made up words.
I hate to say it, but by the time students get to high school, they have often lost the ability to be so imaginative. That's why I like this one better in elementary school.