How would know if my child needs dental x-rays?

Individually based . That would depend on the age of the child, whether and when any images had been taken before, history of problems with the teeth or any clinically observed needs. Without dental images areas of decay, pathology or extra or missing teeth could not be diagnosed. X ray hygiene is very important in the dental office.
Dental Radiographs. That would be determined by the date of the childs last radiographs. The age and the reasons for the radiographs would need to be explained at your next visit to the dentist. Dentists can not diagnose properly without them. It would be like flying blind.
Dentist. Your dentist would advise you on the need for x-rays.
Ask the dentist. Children's dental x-rays are taken to asses tooth, jaw, and skull development. They are also taken to clarify a developmental or disease process. 1 day up in the mountains skiing, or 2 days on the beach, is less radiation than a full mouth series of 14-20 films. Your dentist will do everything possible to minimize x-ray exposure, but the films are necessary to visualize hidden areas.