def writeToChild(childFD, data):
    """
        Similar to L{ITransport.write} but also allows the file descriptor in
        the child process which will receive the bytes to be specified.

        @type childFD: L{int}
        @param childFD: The file descriptor to which to write.

        @type data: L{bytes}
        @param data: The bytes to write.

        @return: L{None}

        @raise KeyError: If C{childFD} is not a file descriptor that was mapped
            in the child when L{IReactorProcess.spawnProcess} was used to create
            it.
        """
