An item by Denise Mann on WebMD.com, updated on May 10, 2005, quotes ACSH's Dr. Ruth Kava:
Ruth Kava, PhD, RD, director of nutrition at the American Council on Science and Health, has this to say: "The bottom line is not to restrict them, but to help them grow into their weight because children need extra calories to grow. Don't put kids on a strict diet because they are probably going to resent it."
...But whatever you do, say no to weight-loss supplements for kids, Kava says. "You don't really know what's in them, and most have not been tested in kids to determine their safety or effectiveness."
See also: ACSH's What's the Story pamphlet on Drug-Supplement Interaction.