I have dealt with this problem many times, so here's my advice.
Use Advantage on momma, being careful that it's allowed to set into the back of her neck for at least a minute with her upright, this way the oil won't just drain/settle to one side of her, which can enable the babies to come in contact, or even get to their mouths when searching for mommas nipple.
After a couple of days you may notice the fleas are not so much around any of them, mom or babies from her treatment.By
the way, I am very confident that Advantage truly is the best! Tried the rest...
Babies can have baths at 5 days, and I will explain how this is properly done, I have had to do it quite a bit, as I have rescued newborn litters or abandoned litters from outdoors where they had tons of fleas and no other way around it. However, you might want to prolong this event at least until you feel more secure about doing this.
The keys here are the water temp, and avoiding chilling them afterward.
Run the hot water first, for a long enough time that you know it's the hottest it will get.
Now add the cold, and get a good feel of it, testing on your inner wrist, anyplace other than your fingers where it's a bit more sensitive.
You don't want it too hot, but not cool in any way!
Place the baby across your left hand (assuming you are right handed) where it's head will sort of hang over by the base of your thumb, and it's butt/tail hangs a bit off the other side. It's tummy in your palm.
Put it under the water on it's back first, then wet the neck area. This will be soaped first, as fleas head upward to hide in their face and ears, which you want to avoid if possible.
Make a soapy ring around it's neck. Then wet and soap up the rest, body, under all legs, under the tail, etc.
If you follow this way and soap the neck first, there is no reason to wash the face, so you can avoid water/soap in the eyes and mouth.
The key in flea removal is a long rinse, it technically takes up to 15 minutes for a flea to drown, they do not swim.
So the longer you rinse, be sure to get the underside well, the better chance of removing all of them, after 5 minutes or so a flea will finally let go if it's been watered for this long, and you can see them washing down the drain.
I use a heated towel to dry, warm it in the dryer or microwave.
Dry off as well as you can, all the excess water fluffed off.
Depending on how many you will be bathing, sometimes I put the baby right in with mom, she will lick and dry the kitten. Sometimes I have a litter of 5 or 6, so I will put a heating pad, prewarmed into a carrier so that the freshly washed baby stays warm until they are all done.
As I said before, if you treat mom for fleas, then the problem might go away, at least enough for them to grow for a week or 2 before a bath, but fleas can be very dangerous/deadly for a small baby kitten, depending on just how many there are.
Oh, you might see blood on the baby while bathing, this is from the fleas, the more blood water you get, the more fleas there were. Don't be alarmed, I know I sure was the first time!