E.D.
You can do a GOOGLE search for Chicago Talent agencies and a list will come up. You can also buy something called the Act One Reports (call Act One Studios) which lists all the talents agencies and their requirements for submissions. There's also a book that you can probably find at your library called THE KIDS BOOK: A Guide to Acting for kids in Chicago.
Do NOT respond to any solicitation you receive randomly in the mail, or attend a giant "open call" where they tell you your kid is amazing and then say they have to have professional pictures taken and ask you to pay fees to join their agency. Be wary of anyone who promises to make your kid a "star". These are scammers who prey on the vulnerabilities and pride of gullible parents. And there are LOTS of them out there.
For your child to work, you must have a flexible schedule and be willing to do a lot of traveling on quick notice to downtown Chicago. There are lots of auditions and lots of waiting around. You might go to auditions where they are seeing 200 kids and you just have to wait your turn. So your daughter in addition to her looks needs to be patient and have a good and open personality. My kid is totally photogenic but if he had to wait for any length of time (and trust me, I'm an actor/model - there's LOTS of waiting) he'd be a basket case. He just doesn't have the temperment that's required.
Also they make sample sizes for kids in size 3T. So if your kid isn't a normal clothing size the odds are stacked against you.
But if it's just you and your girl with nothing to do and she seems like she could do the job then by all means pursue reputable agencies who handle kids and don't make empty promises. And don't shell out money. Period. Quality snapshots should do at this age.
Good luck!