Currently, experts don't really know if energy drinks are bad for kids, but we don't know if they're good, either. Caffeine is the main active ingredient, but these drinks may contain substances such as taurine, B-complex vitamins, and herbal supplements.
Children and teens should keep their caffeine consumption to less than 100 milligrams per day, and since there is a lack of safety information, frequent energy drink consumption should be discouraged.
Like regular soft drinks, energy drinks are mostly water and sugar, but they contain much more caffeine -- up to three times the amount of caffeine found in cola. The amount of caffeine in soft drinks is regulated, but some energy drinks are marketed as dietary supplements so they can legally have larger amounts of caffeine than if they were sold as soft drinks.
According to published research, small doses of caffeine can have positive effects on children by improving attention, but caffeine can also increase blood pressure and cause sleep disturbances (especially late at night -- try a bedtime snack instead of an energy drink). And more is not better. Larger doses of caffeine may reduce physical reaction time.
So, the full danger of caffeine and energy drinks isn't completely clear. Potential areas of concern for kids who consume large amounts of caffeine from energy drinks include cardiovascular effects, potential effect on bone health, and possible interactions with medications used for attention deficit hyperactivity disorder. The excess sugar may lead to weight gain and obesity and put the child at risk for developing cavities of the teeth.
Cinteza E. "Update in pediatrics: to take or not to take soft drinks, sports or energy drinks?" Maedica (Buchar). 2011 Apr;6(2):157-8.
Seifert SM, Schaechter JL, Hershorin ER, Lipshultz SE. "Health Effects of Energy Drinks on Children, Adolescents, and Young Adults." Pediatrics. 2011 Feb 14.