My brother has been in many print ads and is a professional actor and comedian. When we had my first son, we looked at getting him into ads and sought his advice.
First, realize this should be a hobby. Take it all with a grain of salt because there is a ton of rejection, even for the most seasoned and well-known actor/model.
Second, get some great shots of her. Sure, professional shots are nice (Sears, Penneys, Walmart, etc), but so are impromptu snap shots. Get some close ups of her face and also a few of her full body. The agency we used (one of the biggest and well known in Texas) preferred to have more snapshots than professional shots. This was due to the ability to airbrush and touch up professional shots. AND, you can really see personality in a snapshot. We were blessed with having an "in" with this agent due to my brother and got a lot of great advice.
Third, keep a record of her measurements (including weight) and clothing sizes. Agencies want this information to help find her work that is appropriate for her size and to match her with what each company wants age/size and such.
Find a reputable agent. No fees will ever be required. There are many in the Dallas area (where my brother started his career prior to making his big move to CA). Make sure they deal in children (some agencies don't specialize in children and, thus, don't get calls from many companies). Send in her shots and wait. It can take quite a while to get a response.
Last, most people don't realize this, most children under the age of a year old are not models or actors. They are the directors child/grandchild, the adult model's child, someone on staff's child/grandchild/niece/nephew, etc. So the amount of real business for non-related children is minimum.
My brother's biggest piece of advice: when the child says they don't want to do it, it stops completely. No more casting calls, no more pictures, no more ads.