For a healthy diet, added sugar isn't necessary at all. In fact, the USDA and USHHS Dietary Guidelines for Americans 2010 say to reduce foods and drinks high in added sugars. "Sugar-sweetened beverages provide excess calories and few essential nutrients to the diet and should only be consumed when nutrient needs have been met and without exceeding daily calorie limits."
The best way to determine how much added sugar is acceptable for your child's diet is to talk with their doctor
or a registered dietitian. A doctor or registered dietitian can help you determine how many calories your child
should be getting a day and how much added sugar is acceptable.
U.S. Department of Agriculture and U.S. Department of Health and Human Services. (2010). Dietary guidelines for Americans (7th ed.). Washington, DC: U.S. Government Printing Office, December 2010. http://www.cnpp.usda.gov/publications/dietaryguidelines/2010/policydoc/policydoc.pdf