I would go to an excellent photographer who specializes in shooting children and specializes in comp cards/headshots. I would get a few comp cards/headshots done and submit them to some agencies in your area. If the photographer does a lot of comp card work, he or she may have some agencies they can suggest.
You often hear, "Just send in a few snapshots" -- but I've found it's MUCH better to have a professional comp/headshot to present to agents. And it's great if you own those images and have them on a disc. Just a warning that you will be paying for comp/headshots anyway. That's considered a "professional expense" that you pay for.
Note: Agencies may not work with you if you're too far away -- some will, some won't. They also LOVE a flexible schedule -- is your child homeschooled? Is one of the parents a full-time stay-at-home parent to provide transportation? That helps.
Don't feel bad if you're rejected by most agencies. Some agencies may have enough kids in that age bracket, they may have a child who already has your kid's look, etc. If you get accepted at one or two agencies, that's great.
A couple of other tips. Legitimate agencies don't require you to pay for "modeling classes", they don't charge you a "representation fee." They don't force you to use their photographers. You shouldn't walk in the door and pay anything. There are a lot of scams out there.
The only legitimate expenses you should pay for are 1) comp cards and headshots (redone every year to two years), 2) you may pay a monthly subscription fee to be on an external modeling website.