The best way to find a good, reputable agent is to network with other agented authors and see if one will read your manuscript, and if they like it, they might be willing to recommend it to their agent. The best way to do this is to attend conferences and writers' workshops. Writing for the children's book market is seldom as lucrative as writing for the adult market. Most picture books sell from 5,000-10,000 copies in hardcover and go out of print within fourteen months. Few go into paperback.
Easy readers are about the same.
Read your contract very carefully. In the end, writers write because they love creating stories and making words sing, and they love to see children enjoy the stories and books and poems and articles they have created. These are the true riches that a writer for children will garner. You don't have to be published to achieve this goal.