How about starting with AA. Milne's When We Are Six - old fashioned, but sweet and like little stories and quite a good introduction to poetry. I think there's also a The Nation's Favourite Poems antholgy for Children, which might be good.
Funny, I was thinking of asking dinosaur that - some postings about our respective dss liking poems & think she's rather knowledgeable. Anyway, we got given this (though annoyingly I see it's unavailable at Amazon) & there's lots in there that ds & dd both love. It'll be in the library. Is this Easter hols homework?
In support of poetry, there is the rhythm thing - in the Oxford Anth I linked to there is Auden's "Night Mail" and reading it aloud very fast so that you can hear the train barrelling along is completely different from listening a story in prose (and I think more fun, but there you go).
coddy, dunno if I'm too late on this one but have you come across Allan Ahlberg's kids' poems? Please Mrs Butler is a hoot, the review says ages 8+ but infants would appreciate it too I think (suits 50+ as well...)
Another vote for the one wilbur recommended BBC The Nation's favourite poems of childhood We've got this, and it seems to have everything in it that I remember and enjoy from childhood ... everything from Spike Milligan to Hilaire Belloc to Roald Dahl to A A Milne. A really great selection.