How do you know if your dog and cats do not have fleas? (The areas that you are describing are the most common areas that fleas congregate.) Take a wet paper towel and rub through the hair coat (especially above the base of the tail) if your paper towel comes away red or pink your pet has fleas. Fleas feces is blood. If one pet has the fleas you will have to treat all your animals.
Here is information on treating for fleas:
http://www.ca.uky.edu/entomology/entfacts/ef602.asp
The fleas were horrendous this year!