My theory also, you just said the same I been saying.. Been awaiting the finale..

Actually, I did notice some things before I read the text. First was the angle of the blades. I could draw the blades across my arm and it would leave scraggly lines and not deep enough to bleed to death. and second, where did the open space come from when the "painting" drops down ? I see where the cuts start that don't match with the girls' feet where they removed the frame from the wall.
Pretty shoddy "magic" trick. Maybe it would work for your kids' birthday party.