Look in your local area for agents or you can always use beauty pageants too, a agency can discover them there. Usually you do have to pay to put your child in contest but for an agent to pick them up you shouldn't have too, they usually take their percent when the child works a job. You will have to pay for head shots or you can do them yourself and save.
A friend of mine would send a photo to catalog places and never paid to have her kids model. Her son met even went to an event were he took a picture with oprah.