Accessing files in a child directory in Python 2.7.x
I suggest a generator:

def children_files(dir):
    onlyfiles = (f for f in os.listdir(dir) if os.path.isfile(os.path.join(dir, f)))
    for f in onlyfiles:
        file_path = os.path.relpath(f)
        yield os.path.join(dir, file_path)


It is simpler to write (no append) and faster as files will be read just as needed.