One of the conditions that's often missed in kids is sleep apnoea. It is a condition where the child might stop breathing during sleep. Often symptoms of such a sleep disorder include heavy breathing, snoring, mouth breathing, and brief pauses in breathing during sleep witnessed by parents. From your description, it seems like your child has some of these symptoms, so the possibility that she might have this condition. I suggest you see a sleep specialist or a physician who can evaluate for sleep apnoea in kids for further evaluation/tests/diagnosis and treatment.