Even if the comb won't get rid of all the fleas on the Mom very easy, I would follow Gayef's advice over the vet. I know that sounds goofy, but my vet recommended and over the counter flea treatment on the Mom of 4 week old kittens. He said just put it up on her shoulders, and it would be safe. I posted here first, and was advised not to use the otc on the Mom. I had thought it seemed like the kittens would get too much exposure!
My vet is a good vet, but not a kitten specialist. I have a feeling there are many vets like him out there...I would never use a strong chemical on or near a human baby, and kittens are very fragile, too!
I trust Gayef...she has raised many kittens. The comb will help a lot. It may be more work, but if you keep combing the Momma and the kits, it will keep the fleas down.
Generally, if they have fleas, they have worms, too. So check with your vet on that...do they need a poo sample, and when can the Mom/babies be treated.
Best of luck to you and the babies.