For purely elective surgery I would postpone any child who has been chewing gum. The act of chewing not only results in large amounts of saliva production (and swallowing) but also gastric fluid secretion by the stomach. Chewing gum increases gastric volume and decreases gastric pH. It increases the risk of dangerous aspiration.
No,I have no evidence for this except basic physiology. However, from a medicolegal point of view, you would be crazy to proceed. Imaging if the child did aspirate (whether or not the chewing gum contributed). How could you defend yourself?