This is the key to breaking into the poetry market. You can't get a feel for the market that will buy your poems unless you read into it. Spend some time at a local bookstore, and browse other sets of children's poems in order to see the different types that are out there. Order children's magazine, and take a look at what makes the cut. There are some areas that will always be of interest to publishers for poetry.
Some publishers prefer easy reading, other prefer non-fiction and teaching books, and so on.
Read up on this. Make your Work Challenging Children are very intelligent. Many writers feel a need to make their work very simple. Allow children to think while they are reading. Also, provide text that will lend itself to rich illustration, because most children love pictures.