The most important factor is if the child wants to act. Then you may want to enroll your child into acting lessons, hire an agent, and attend auditions.
You should absolutely make sure that the child wants to act. Start off with something small, like a school play. You could always sign them up for the local theater/drama class.
Find out where there might be some local theater or print work, build up his resume, then take him to Hollywood or New York and start looking for work.
A lot of kids transition from pageants into acting. If your kid is a boy, you should start him at a local community theater, and possibly hiring an acting coach.