Never pay any money upfront. If the offer requests a fee for meeting with your child, to acquire representation, to attend an audition, to attend a meet-and-greet session or any other request for upfront money, then you are likely dealing with a scam. A reputable agent or casting agency never asks for upfront money. The only time an agent gets paid is if your child gets a paying job.
Call the company conducting the search and investigate the background of the company. Check with organizations like the Better Business Bureau to find out if any complaints have been filed against the company. The Internet helps you find scams well before you devote any time or money to them. Just Google the company name and see if the results show any negative news.
Look for legitimate casting calls in industry magazines like "Variety" of "Backstage." These trade magazines may have cattle call notices for children where legitimate casting directors or agencies are looking for children for specific roles.
Never sign with an agent that tries to sell you things, even if he doesn't ask for upfront money. If an agent has photo sessions, "talent cards" or acting classes that he consistently tries to sell to you, he is not a legitimate agent.
Ask an agency searching for new child talent for references. It should be able to provide examples of specific talent on the roster that have worked recently and also show you what the actors have worked on. An agency that has child actors that are working and auditioning regularly could conduct a legitimate search of some kind to expand their roster, and could be worth doing business with.
Look for searches conducted by actual casting agencies. Go to IMDB.com and look up the casting director purporting to do the search. If they have several IMDB credits, it shows that they are casting work regularly and could be offering a legitimate audition opportunity.