Bad breath or "Halitosis" in children can occur due to several reasons like poor oral hygiene, mouth breathing, tooth decay, gum disease, infections in the mouth, bacteria on the tongue, dry mouth, tonsil stones, foreign objects in the nose and some medical conditions. Start with improving oral hygiene by ensuring you clean the mouth and tongue as well. Take baby to see a dentist to exclude oral infections, decay, gum disease etc. If it's still persistent you may need to see a paediatrician as well to exclude other causes.