The Bundesnetzagentur, or Federal Network Agency, said in a statement issued Friday that watches that would allow parents to "listen unnoticed to a child's environment" constitute an unauthorized transmitting system.
The agency said parents have been using watches marketed to children between the ages of 5 and 12 to monitor teachers.
It didn't name specific brands, but advised schools to be on the lookout for such devices.
The agency also says that if buyers of the products become known to authorities, they will be told to destroy the watches.
The Bundesnetzagentur regulates telecommunications, power and other networks
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