If you mean childrens book for like elementary school age then I think you can just submit directly to publishers--although you should check to verify this.
If you have a young adult book which is more like a chapter book for middle school and up, then you need to find a YA agent who likes the genre of your story (i.e. fantasy, contemporary).
Go to the bookstore and look at some of the books they have on publishing--that would be a good place to start.
I think the Absolute Write.com forums have a children's section as well.
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