You could also use "place them in a bigger box" as a starting task, then introduce the symbolism after, by asking them to choose the correct symbol, while keeping the larger and smaller boxes on the screen to give an additional visual clue as to the meanings of the symbols.
Here are some sample pictures to give you an idea of what I mean:
FYI, I am not a doctor nor have a I ever played one on TV, but I think this may have some merit and seems easily testable. We would need to connect to experts in the fields to come up with good tests. In any case while far from perfect these could act as early indicators of possible issues.