The area of square $ACEG$ can be calculated using geometry of regular polygons. The side length of the octagon is given as 6 cm, and using geometric properties of a regular octagon, we calculate the area of the square formed by the diagonals. Using trigonometric and geometric formulas, the area of square $ACEG$ is found to be:
\(\boxed{36(2 + \sqrt{2})}\)
Thus, the correct answer is Option (4).