The best and most gentle on the market and can be used by the entire family is Dr. Bronners pure liquid castile soap. He even has some scented ones in eucalyptus, tea tree and lavandin & lavender........all of which keeps fleas off. This shampoo is so gentle you could use it every day, and also gentle enough for new born babies.
http://www.dr.bronner.com/drb_sai_liquids.html