How did they do this in 1989?

How did they pull off this giant kid effect in 1989? Niko's best guess is that they used a half-scraped mirror, which made it part reflective, part see-through. This would have allowed the camera to film the kid directly through the clear section, while the adults and the rest of the room would appear reflected in the mirrored section, creating a forced-perspective composite ALL IN-CAMERA. Do you think they did it differently? And if you worked on this movie, we'd love to know how it was actually done!