If they are nursing off the mom well you probably don't need to feed them. If you do feed them anything, do not give them cow's milk, get some milk replacement (KMR, etc.) from a pet store and read their directions.
The only time you would supplement is if the kittens were not thriving. Mother will take care of their need for milk. Between 3 and 4 weeks you can give them some pureed meat. Until then, Mom will do the work.