Most children’s book agents represent young adult and middle grade novels today. A few will also represent picture books, beginning readers, and chapter books. Because of the competitive market these days, most are fine with simultaneous submissions as long as you let them know.
Some children’s book agencies may have several agents who represent the genre you’re interested in. The proper etiquette is to submit to only one agent at the agency you’re targeting. Many agencies hold team meetings regularly and if an agent reads a manuscript that she feels would be a better fit for her colleague, she will share it with him. However, if the initial agent rejects your manuscript, it is perfectly permissible to wait several months and then resubmit it to another agent at the same agency. Or, if a new agent joins the agency, you may resubmit your manuscript to him.
Networking is key. Some top agents want referrals. Get plugged into your local Society of Children’s Book Writers and Illustrators (SCBWI at http://www.scbwi.org). SCBWI is the most prestigious and recognized organization for children’s writers in the world. All the top authors and illustrators are members. Join their ranks. Attend writers’ conferences both at the local and national levels and make friends with successful working authors. Dialogue with your writer friends and let them know you’re looking for an agent. Agents are people and if one of their clients refers you to them, they will take a longer look at your package because of the personal investment they’ll naturally feel.