For most children, the inability to urinate is caused by dehydration. Common causes of dehydration are not drinking enough fluids, diarrhea, vomiting, and fever. A child can also quickly become dehydrated in hot weather.
When a child is not dehydrated, the inability to urinate can be caused by:
A child who is not dehydrated but cannot urinate should be evaluated by a health professional. If the bladder fills up, the child will be very uncomfortable.
WebMD Medical Reference from Healthwise