What causes bad breath in a child who has healthy teeth?

Age,hygiene,diet: Age, diet , and hygiene are major factors in some sources of bad breath. Sinus issues can cause malodor. The usual source is improper or infrequent brushing and flossing. The parent should supervise or even intervene if the child is non compliant.

Digestive: Oral bad breath may be caused by a digestive problem. This can be related to the types of eaten and/or how the body breaks it down. See your pediatrician.

If oral hygiene: Is being well addressed by the parent for the child, and not by the child only, then an evaluation by the child's dentist and physician might also need to be entertained. Just because the child's teeth might appear white, doesn't preclude the fact that there might be oral issues. Hope this helps.