Generally it is not harmful if the child swallows a chewing gum once or twice. Children under 5 should not be given any chewing gums because they don’t understand the difference between swallowing and chewing. In case if they have already swallowed, there is not much to worry. A chewing gum has a gum base which is indigestible. But it doesn’t stay in your child’s stomach. It moves relatively intact through the digestive system and is excreted via stool in a day or two.
On very rare occasions, large amount of swallowed gum can combine with constipation and block a child's intestine.