It is quite common for children with cold and flu to switch between nose and mouth breathing, as nasal blockage makes it hard to breathe through the nose. This is usually temporary and improves as the cold settles. You can help by keeping her well hydrated, using saline nasal drops, and steam (if age-appropriate). However, if mouth breathing continues even after the cold improves, or if she has difficulty breathing, snoring, or poor sleep, it is best to consult a doctor.