While there may be a few who have used certain products off-label with success, I am just not comfortable with it. There is more to it than it just wasn't tested that way. Many cats have reactions to the spot-on flea treatments and if you put it on kittens that young, it is a certainty that Mom is going to lick them and ingest the chemicals.
Hissy's advice in the post that TNR1 left is, in my opinion, the only safe way to remove fleas and ticks from the babies. Do NOT bathe them in Dawn or any other kind of dishwashing liquid ... the risks of hypothermia and other issues are far too great! I cannot fathom that a vet would recommend that!!!
You must be very persistant in tweezing, combing and making certain you've removed as many of the fleas and ticks as you possibly can several times a day. This also has the added benefit of allowing you to bond more closely with the kittens in that they come to recognize your smell, your touch and your sounds.
Best of luck,