This article may help: http://www.dallaschild.com/showarticle.asp?artid=676
My daugther is 21 months and models for Kim Dawson. Just like the other post said, you just send some pictures (they do not have to be professional) and if they are interested, they will contact you. Also, you should not be required to pay for anything like classes to begin. Also, pay attention to each agency, some want snail mail and others email. I sent my daugther's pictures to four agencies. I heard back from two, one said no, and Kim Dawson said yes. The turn around was very quick with Kim Dawson, I think I sent her picture in on a Monday or Tuesday, I got an email that Friday for an interview with her the next weekend, then about a week later she received her contract.
The one down side to your daugther's age is when they are called for jobs, it will be during the school day. A friend of mine has a six year old that has modeled for the past year and a half, now that she is in school, she has actually had truancy charges filed on her. Even if her daugther has only been gone for a couple of hours during the school day, it counts against her. BTW, my friend and I are also both teachers and often times they (the agent) will call you the afternoon before your child is needed. This can be a hassle if your spouse can not take off and you have to get a last minute sub and make plans for a sub. You do not have to say yes to every job, but when your child is first starting you feel like you need to. The jobs are located all over the Metroplex.
Best of luck,
S.