I'm a bit of a expert on the subject due to the fact that I had to get these same critters out of my grandmothers fireplace a time or two. Depending on the situation, it is not as difficult as you might think. Assuming you have reasonable access to the roof, and it is not an insane length of chimney, here is what I did.
Get some nylon rope about twice the length of the chimney. Then get some rigid tubing the length of the chimney. I used 1/2 inch emt conduit. The nylon rope works good because it is not very flexible and this makes it easier to make a snare. Find the midpoint of the rope and fish it down the tubing to make your snare. I had an extra set of hands to hold a good source of light so I could see what I was doing. Once the snare is in place down the chimney, it is just a matter of fishing them out one by one which is easier said than done.
It is best to wait until the little (or whatever you call them) is to the age that the mother does not spend as much time hovering around the area. They can be hostile if they see what you are up to especially if they are really young.
P.S. The babies make good pets until they get old, turn mean, and try to scratch your eyes out of your head.
