Do not use flea baths on any cat, especially kittens.
If you mean just a regular bath, you can fill a loaf pan with soapy water, and then run the flea comb through their fur, and then dip the flea comb in the water, rinse and dry it, and repeat. (The soap keeps the fleas from escaping).
It is also super effective to treat the environment, wash every thing in hot water, vaccuum daily, etc.
Some flea meds (prescription) are safe to use on younger kittens, you could ask your vet about it!