It is ultimately the responsibility of the imaging professionals (radiologists, technologists and medical physicists) to develop proper imaging protocols to suit the age and sizes of the patients being imaged, not only from a radiation dosimetry point of view, but also an image quality point of view.
For a long time now the tendency, particularly with CT imaging, has been to use a one size fits all protocol for everybody.
The move to reducing medical radiation exposure has been going on for several years now. Recent high profile incidents (such as what happened with CT patients getting CT angiograms at Cedars Sinai) prompted many institutions to review their imaging protocols. The Image Gently campaign has worked towards getting institutions to develop pediatric specific imaging protocols for several years now. For procedures involving potentially high radiation exposures, radiologists review and protocol the requests to make sure the requested exam fits the clinical indications. Outpatient imaging centers performing CT imaging need to get their scanners accredited in order to get Medicare reimbursement. Accreditation by the ACR (American College of Radiology) places dose limits on several types of scans. If a site's protocol results in a radiation dose that exceeds the limits, they fail accreditation and need to adjust their protocols.
The FDA can and should require manufacturers provide the tools to enable imaging facilities to monitor and record the amount of radiation given during a procedure, but it needs to be the responsibility of the users to make sure the imaging equipment is used properly on patients.