I rescued a litter of kittens several yaers ago about the same age and they also had fleas. My vet had me give them a bath in the original blue dawn dish soap. I put about three inches of water in my kitchen sink with a few drops of the dawn then I rinsed them. The important thing is to keep the kitten warm so as soon as a kitten was rinsed we wrapped them in a warm towel for a few minutes to dry. The dawn will not kill the fleas but it stuns them so the next step is to use a flea comb and to comb the kitten. Since the fur is still a little damp it is easier to see them and i had a bowl of water i would dump the flea in as soon as I found them.
The adult cats need to be treated with frontline and any other pets in the house. My other advice is to vaccum often, if you have a vaccum with a bag get a flea coller and cut it into smaller pieces to put into the vaccum bag so any fleas in the bag are killed. It was quite a process but it worked. The dish soap must be the blue original dawn only, the vet said any other soap can harm the kittens. We had a box with a heating pad and a towel over it, we washed one kitten at a time and put them in the box to stay warm until they were dry. Good luck!
