What to do when a rude neighbor comments on your child's behavior?

My neighbor has made repeated comments about my 3 year olds behavior, saying he is just so out of control and he wouldn't know what to do if he was his child. I have 2 children, a 2 year old and a 3 year old my neighbor only has one 5 year old child. Her little girl is extremly bratty and they just don't seem to see it. How should I react when they say things to me.