The sudden greying of the hair in children could be due to high levels of stress, which can lead to the production of free radicals that cause damage to stem cells that produce pigment.
A quick google search also shows that you are wrong about stress not being able to cause gray hair and the article is accurate. Though it disappears when the stress is gone
Though the legend is inaccurate—hair that has already grown out of the follicle does not change color—a new study(link is external and opens in a new window) from researchers at Columbia University Vagelos College of Physicians and Surgeons is the first to offer quantitative evidence linking psychological stress to graying hair in people.
And while it may seem intuitive that stress can accelerate graying, the researchers were surprised to discover that hair color can be restored when stress is eliminated, a finding that contrasts with a recent study in mice that suggested that stressed-induced gray hairs are permanent.