A poet friend of mine, Liz Brownlee, is putting together a very useful and interesting listing of poets who write for children. Like myself they don't all write exclusively for children but will all have had children's poetry published in their own collections or in anthologies. The list is extensive and includes well-known names like McGough and Zephaniah as well as many far less familiar - there are also quite a few poets from overseas, writing in English. Many of the poets are available for hiring for school visits and the like, and the listing includes contact details for them. Have a look, you can reach the site via Poetry Roundabout. I recommend it because each poet has selected one of their own favourite poems and overall you get a flavour of the richness and variety of children's poetry available today.
When Liz asked me for a favourite poem to include I found it, of course, extremely difficult. In the end I plumped for A Poet is for Life which is really for older children (and adults) but I thought it would amuse readers and be a bit different from the usual children's poem (if there is such a thing). You can read it on Liz's listing.
One other I had considered was Now I'm Not a Little Kid which was first published in a Macmillan anthology of 2005 called Monster Poems edited by Brian Moses. I still like the poem a lot and I included it in my Hear Here! collection for children. Lucy drew a lovely little illustration for it - here are poem and illustration.