Doctors say a child has a bed-wetting problem if the child is older than 5yrs and wets the bed 3 or more times a week. The proper name for this is nocturnal (night time) enuresis (inability to control urination) and it is a very common problem.
Children who have never been dry are said to have "primary" enuresis. If a child starts wetting having been dry for some time they are said to have "secondary" enuresis. In the vast majority of cases there will be nothing else wrong with the child.
There are several factors which are thought to be important in enuresis all of which are completely outside the control of either the child or the parent.
The most important thing is to understand that the child is not wetting the bed "on purpose". He or she does not want to wake up with wet bed clothes and desperately wants to be like other children. Other than excluding urine infection or diabetes and giving you general advice your doctor will not "do" anything until your child is at least 6 yrs old.
If your child is still wetting the bed after the age of 6yrs then the doctor will probably refer you to a special enuresis clinic where you will be advised about ways of dealing with the problem including alarms and medicines.