Many people are under the impression that you have to wait 3 days or 72 hours before you can report a person missing. This is not the case. It doesn't matter if it is a child, or an adult, you can call the police and file a missing person report as soon as you know they are missing.
It most cases the criteria to file a report is that you must be a close family member, but this would not be the case if it is a child missing. For instance if you are a babysitter and the child goes missing, you would call the police immediately.
You should also consider before you file the report it they are really missing. Sometimes people will disappear for many reasons. They are mad and need a cool down period or they are depressed and want to be alone. It is not against the law to be missing, so you may find the police will not be in a rush to locate them as you are, unless there is a circumstances where the missing person is in danger or is a child, or because of the recently implemented "Silver Alert," if they are a senior citizen.