Depending on the childs age photographing babies is very not a difficult skill. As they get older it takes more skill to get their attention and looking good so it really depends on their age I guess. With regards agencies I like Scallywags and Norriecarr, personally I wouldn't pay for pictures of the child unless its for you, but childrens agencies will charge a fee for being on the books, this is normal but going to castings with them will earn the gardian a fee, usually about £15 (for travel).
I shoot babies day in day out if you are interested and do not mind a bit of travel.
Mc GIllicuddy is bloody good of course but would not be cheap unless you just happened to pickt he time he was running a course. From talking to previous models I've done this for you do not need anything too special . The important part is updating on a regular basis as babies change so fast.