I work in the modeling industry (with the women and men). For kids, the 2 best are Ford and Wilhelmina. After that, I think Rachel't Tots and Teens, Funny Face kids, if Gilla Roos has a kid's division, they're good as well. I think that if you go to any of them, they actually have a list of the other ones (if they don't take you).
Ford and Wilhelmina def don't make you "buy" pictures. If they take you, you just leave snapshots with them.
It is not as easy as people think. You drag your kid to a casting, wait with a ton of other kids and parents, they take a picture of the child. And then you "might", a big "might", get the job. They also have laws on how many hours a child works. Babies can't work more than 2 hours a time, not sure what it is for toddlers. So, if you're in the city (Manhattan) it's a lot easier than if you're driving in from someplace else.