A: Dear Babysitter:
It is damaging to your son both physically and emotionally to have him suffer bites from another child. Human bites are also liable to bad infections, because of the bacteria which are in everyone's mouth cause trouble when they are embedded under the skin. You need to take strong measures to separate the two children, even if you are only turning your attention aside for a moment. You can use playpens, kiddie-chairs, or a number of other toys and devices to keep the two children entirely separate while you attend to a sippy cup or make a phone call or whatever it is that you need to be doing.
Both your son and the little boy who bites need you to say clearly and firmly, "No no--no biting!" But you have to enforce this at all times, by not letting the child succeed in biting.
You should also communicate with the parents of the child who bites. It might be natural for you to feel angry at them, but try to remember that it is not their fault that their son bites. They need to know that this is a problem, so that they can be watchful with the little boy when he is at home with them. Perhaps he is too young to be left with a babysitter or perhaps he is reacting to other stresses in his life.
If you are simply unable to prevent the biting by separating the children, you may need to explain to the other child's parents that you cannot babysit this little boy until he has stopped biting.
Biting is common among babies of this age, but it is more serious for your son--who is too little to understand and who can be traumatized by being bitten. He needs to feel safe.
Elizabeth Berger MD Child Psychiatrist and author of "Raising Kids with Character"