Teeth whitening has come a long way over more than twenty years. Initially we were concerned that the chemicals used to whiten teeth would cause some sort of permanent damage to teeth. Nothing is farther from the truth. Many people have whitened their teeth without serious concerns, in fact teeth whitening has become one of the most popular cosmetic trends (as demonstrated in all of the Over-The-Counter teeth whitening products and myriad toothpaste formulations).
Doing cosmetic dentistry for children always strikes some controversy. In my opinion, that's more voodoo than science. In general children CAN have teeth whitening at an early age. In my opinion a child COULD have teeth whitening done even at age 11, depending upon how their teeth are growing in. As with any kids that age, the cooperation needed to actually complete the whitening depends a lot on the individual child and the overall supervision of the parents.
Candidly, I do not get many requests for teeth whitening for young kids. Rather, mostly for young adults who often want to whiten after they finish their braces or teeth straightening.
For young children the main aesthetic treatments typically are bonding to fix chips in front teeth, or to close up unattractive spaces.
In terms of techniques, there are two that can be useful: one, professional tray whitening done at home, and two, in-office professional whitening. Either technique depends a lot on the child's maturity to help out with the process.
In summary, I do not see a strong trend for child teeth whitening. I believe it CAN be done safely for even young children (say 10-12 years old). There are at least two different techniques that can produce the results. In some cases, a child could have it done at even a younger age, to just treat the front part of their smile.
I have worked with the media before and would be able to provide you with a very strong interview if you need more information.