this wont hurt a bit

This wont hurt a bit is a common idiom which is often used to explain to children that a certain thing wont hurt them. It is very common to use when speaking to children about pending medical procedures, such as a shot; while a shot hurts, it still helps to tell them it will not hurt very much. While more commonly used when speaking to a child who is worried about the possible pain of something, it may also be used in situations with adults.