If I am making a salt water gargle for a child who sometimes drinks the gargle. How much salt is safe to use?

Not needed. I would advise against it since it has little or no bearing on health and he might become addicted to salt as a taste.
1/4 teaspoon. The standard amount is 1/4 teaspoon per 4 ounces of water. If he/she drank a little of that it would not hurt them.