Hi nemo, sorry, i'm in london, so only know of london agencies....
agencies dont only take on the impossibly cute kids, they also look for something that makes them a bit different, and the most important thing is that the child is happy and relaxed in front of the camera, is happy to be held/spoken to by strangers,wont mind getting having their clothes changed a lot, is confident, ect, if you are interested in your child becoming a model, its best to get a few clear snaps (dont pay for professional photos) and send them off to a few agencies, and wait and see if any take an interest, I wouldnt go with the first one that takes him/her on, much better to wait till you have had replies from all the agencies you have applied to.
Agency fees vary a lot, there are 4 in london that have very low charges, my dd1 and dd2 are with Urban Angels, and they charge £40 for a years contract, that includes photos every 6 months for the baby and every year for the older one, but there are some agencies whos fees run into hundreds and hundreds of pounds, personally I would avoid these, because you can never be sure if your child will get work, whichever agency they are with
good luck