Like Antigone says, best to check with the vet. However, this is taken from the data sheet for Advantage flea treatment, taken from the NOAH compendium:
It is particularly important to apply the dose to an area where it cannot be licked off. Correct application will minimise the opportunity for the animal to lick off the product.
Apply only to undamaged skin. Do not allow recently treated animals to groom each other.
Use During Pregnancy and Lactation
Treatment of nursing bitches and queens controls flea infestation on both the dam and offspring.
No reproductive toxic effects have been observed in rats and no primary embryotoxic or teratogenic toxic effects have been observed during the studies on rats and rabbits. Studies on pregnant and lactating bitches, queens and does together with their offspring are limited. Evidence so far indicates that no adverse effects are to be expected in these animals.
So it involves some risk. Personally I would probably just use a flea comb until the kits are a bit older- they are old enough to be treated at 10 weeks. I would get them vaccinated for myxi as soon as they are old enough though (6 weeks).