How much sugar should a fifteen month old intake daily?

They should not have any sugary foods or drinks. If the sugar is not an original ingredient (like in fruit), the child should not have it. An occasional sweet (once a week?) would be reasonable. While you are at it, do not feed the child any of the sugar substitutes: nutrasweet, aspartame, high fructose corn syrup, splenda and so on. The fewer additives (color, flavoring) and processing of the child's food, the better.