YEARS ago, we brought in a stray mom that had kittens under our porch. They had fleas also. The kits werent more than a week old. I went to our vet and got some drops for the mom - cant remember what it was but they said is was safe for nursing, and I bathed the kittens in Dawn (like Laureen suggested). Make sure you have mom out of the room and preferably out of ear shot and perhaps distracted with some nice wet food. The kits will cry and this will make mom nervous. If the kittens arent fighting you too much leave them in the water longer as the living fleas wont be able to breathe. Also this can help you pick some of the fleas off easier.
Here's a tip - before you get the kittens wet... put some detergent around their neck - this will help deter the fleas from running into the kittens face and ears when you get the rest of their body wet.
I had a flea comb and a nit comb, but I found a pair of tweezers worked best to pick the fleas off. When the kittens are wet the fleas will lay low and they are hard to get to.
It is important to get the fleas off the kittens as soon as possible. They are very small and the fleas can cause blood loss (anemia).
Treat the mom cat FIRST. She is the source of the most fleas. If you dont treat her first, they will only return to the kittens after their baths. The drops you put on the mom cat will also help kill any remaining fleas that you might miss on the kittens.
Good luck. It is a daunting task, but the kitties will love you for it!