A fifth-grade teacher in Cambridge who is facing a charge of transportation of child pornography admitted to secretly videotaping children changing in and out of their clothes on more than one occasion, according to federal law enforcement officials. Josh A. Wairi, 27, of Somerville was charged today in federal court in Boston in a criminal complaint filed by a US postal inspector. In the affidavit, the inspector detailed a hunt for Wairi through cyberspace, kicked off by tips from America Online and Dropbox Inc. The search eventually led to Wairi’s Somerville apartment, where law enforcement officials executed a search warrant...