Very young kittens are at risk of being poisoned by flea products intended for adult cats. Check with your vet for a suitable product for kittens and use only the recommended quantity. Very young kittens should be groomed with a flea comb to safely remove fleas. This is a tedious but necessary task. All kittens should be treated at the same time as the fleas will jump from one to the other when they nestle together.
Kittens from farmland may harbor bloodsucking ticks; these may have dropped off of the mother earlier or picked up from any grass the kittens have been deposited in (or carried through) by the mother. If you have suitable experience these can be removed using the pull-and-twist technique and special tick tweezers. A vet can remove them safely. While a small tick may appear to do no harm, there are tick-borne diseases. Several large ticks on a tiny kitten may lead to anemia or at least put an additional strain on such a tiny creature.