Bad breath is caused by the bacteria in the mouth. Morning breath when your child wakes up first in the morning is common. However, if the bad breath lasts the whole day, then it can indicate deeper problems. Dental reasons for bad breath include tooth decay, gum infections, dryness and poor oral hygiene. Good oral hygiene practices like brushing and flossing, treating tooth decay can help overcome bad breath. Poor digestion, chronic sinusitis and diabetes can be other non-dental reasons.