The small, shiny, brightly-colored cans are just the right size for small hands. That is one reason why so many calls about energy drinks to U.S. poison control centers involved children younger than six.

Many of these callers asked for help on serious symptoms, including abnormal heart rhythms and seizures, a presentation at the American Heart Association's annual meeting reports.

The nation's 55 poison control centers received over 5,000 telephone calls about actual or suspected use of energy drinks from October 2010 to September 2013. Fully 40% involved unintentional use by young children.

Caffeine poisoning can occur in adolescents at amounts over 100 milligrams a day. That's half a cup of coffee or less for a 45-pound child.

These reports are probably only the tip of the iceberg, since they do not include emergency room visits or the many children who become ill but whose illness is not reported, according to Steven Lipshultz, the study's senior author.

Energy drinks have no place in children's diets, said Lipshultz. Even adults with underlying cardiac, neurologic or other serious medical conditions should check with their healthcare provider to make sure it’s safe for them to consume the caffeinated energy drinks.

Energy drinks vary considerably in the amount of caffeine they contain, with some containing over 400 milligrams per can or bottle. Coffee also varies in caffeine content, though 100-150 milligrams of caffeine per cup is common. Too much caffeine can pose problems even for adults.

Ill effects from caffeine are loosely referred to as caffeine overdose or poisoning. These terms cover a wide range of problems, from minor effects such as nervousness to more serious ones such as a rapid heartbeat.

Up to 400 milligrams of caffeine a day — about three to four cups of coffee — is generally thought to be safe for most adults, but more than 500 or 600 milligrams can bring on many side effects. Some people are more sensitive to caffeine than others; and for them, even a single cup of coffee or tea can cause problems.

Naturally, this is even more of a concern in children. Caffeine poisoning can occur in adolescents at amounts over 100 milligrams a day, Lipshultz says, and that's half a cup of coffee or less for a 45-pound child.

So even if energy drinks and their caffeine aren't harming adults, adults still need to do more to keep children away from them. Energy drinks are just too tempting and when young children get their hands on them, too many bad things can happen.

Steven Lipshultz, MD, is professor and chair of pediatrics at Wayne State University and pediatrician-in-chief at Children's Hospital of Michigan in Detroit. The findings were presented at the American Heart Association's Scientific Sessions 2014.