CPC has long been known to cause staining in teeth. Follow some of the links in the wiki article (to WebMD, British Dental Association, etc.). The Crest rinse has a high concentration of CPC. The anecdotal information tends to indicate that this concentration is leading to a high incidence of staining.
I don't blame people for being skeptical, especially since I'm mostly a lurker here. Being skeptical is a good thing. That being said, make sure that you get all the info about this product before you use it -- or, especially, before you allow your children to use it.
Unfortunately, P&G refuses to put a warning on the bottle. In fact, IMO, this product should require a prescription and should only be used under the care and observation of a dental professional.