Take a few snapshots with your child at home. Doesn't need to be professional. But try to get at least one head shot and one full body shot. Parents think pouts are sometimes cute but talent agents like to see the smile. Keep a plain background and no hats on the head.
Look up all the web sites of the talent agents within an hour drive of where you live. Go to their web sites and find a link for submission guidelines. They usually have different guidelines for different categories of talent. The guidelines will tell you exactly what they want you to send.
Send in the photos with a cover letter and other submission requirements. Do this for as many (at least 5) talent agencies. If they're interested, they'll contact you for an interview and offer you a contract. Get them started early, about 6 months old...it's a great way to let them earn money for their college fund. And despite what the industry seems to be, it's a lot of fun for young children!