A woman who is flat-chested has small breasts, so the front of her body looks rather flat. It is a description of shape, not of health, and not of whether she is a child. It is an adjective: a flat-chested woman, or she is flat-chested. Informal. Though actually, it can sound unkind if you say it about a real person who can hear you — as if her body were a problem. In stories, clothes shopping, and medical talk it is just information. Bodies also change with age, sport, and weight, so “flat” here is a rough look, not a number from a doctor.