Not trying to be gross here...
How long has your daughter had it? My 4 year old nephew had developed halitosis and for two months, back forth with the Pediatrician and Dentist, no one could explain it or find a solution to it. One day my brother decided to look for himself. He checked his nose and mouth. Low and behold, there it was. Something shoved up his nose.
They took him back to the Ped. and he said that my nephew had shoved a piece of Nerf Ball up his nose so far that no one seen it. With time, it had started to come back down the nasal cavity. They were able to pull it out without surgery. It was disgusting; covered in mold and all. Of course he had a nose bleed for a while too.
He was so happy and took a deep breath through his nose and said "that smells better."