Occasional bad breath in children is normal — morning breath, or breath that smells of what they just ate, is not a concern. Persistent bad breath that does not clear with brushing is different and worth investigating. In the vast majority of children, the cause is in the mouth and is straightforward to address.
Poor oral hygiene — the most common cause
The most frequent cause of bad breath in children is plaque and food debris sitting on and between the teeth, and on the tongue. Bacteria in the mouth break down food particles and produce volatile sulphur compounds — the source of the smell.
The areas most often missed are: the back surfaces of the lower front teeth (where plaque builds up quickly), between the teeth (flossing is needed — a brush alone cannot reach here), and the tongue surface, particularly the back third.
If your child's breath improves noticeably immediately after brushing but returns within a few hours, plaque build-up is almost certainly the cause. Improving brushing technique — two minutes, twice daily, with a soft brush — and adding flossing usually produces noticeable improvement within a week.
Tooth decay or a dental abscess
Decay that has progressed to the dentine or pulp of a tooth creates a distinctive, unpleasant smell. A dental abscess — an infection at the root of the tooth — can produce a particularly strong, foul odour, sometimes described as smelling "rotten".
If your child has consistently bad breath that does not improve with better brushing, or if you notice a dark patch on a tooth, a pimple on the gum, or your child mentions tooth sensitivity or pain, book a dental appointment. The bad breath may be the first sign that a tooth needs attention.
Mouth breathing
Children who breathe predominantly through their mouth — often due to a blocked nose, enlarged adenoids or tonsils — develop dry mouth. Saliva is the mouth's natural cleaning and buffering system: it washes away food particles and bacteria and neutralises acid. When saliva flow is reduced, bacteria multiply more easily and bad breath results.
Signs that your child may be a mouth breather include: sleeping with the mouth open, snoring, a consistently dry or chapped lower lip, and waking with particularly bad breath. If you notice these signs, mention them at the dental visit — and, if appropriate, to your paediatrician, as mouth breathing can have causes beyond the mouth that are worth investigating.
Tonsil stones and throat causes
In some older children, small calcium deposits in the crevices of the tonsils (tonsil stones) can cause persistent bad breath. They produce the same volatile sulphur compounds as oral bacteria. Tonsil stones are more common after age 8–10 and are often associated with large or repeatedly inflamed tonsils.
If the mouth is clean, there is no dental decay, and breath is still persistently bad, the throat is the next place to look — a conversation for your child's paediatrician.
What to do
Start here: improve brushing — two minutes, twice daily, reaching all surfaces and the tongue. Start flossing between teeth daily if not already doing so. Make sure your child drinks enough water through the day. Book a dental check-up if the smell persists after two weeks of improved hygiene, or sooner if you notice any tooth discolouration, gum swelling or your child mentions pain.
Sources: American Academy of Pediatric Dentistry, Guideline on Periodicity of Examination, Preventive Dental Services, Anticipatory Guidance/Counseling, and Oral Treatment for Infants, Children and Adolescents, 2022; Scully C, Greenman J, "Halitosis (breath odour)", Periodontology 2000, 2008.