Answer:
Falsifiability.
Explanation:
Piaget's theory is a type of cognitive theory that explains the human development and its nature. The theory mainly depends on the nature of intelligence.
The development of two children of the same age provides the argument against the Piaget's theory. This can be explained on the principle of falsifiability. This principle states that the scientific things must be proven false and it can be explained that include the falsifiability. The cognitive ability of children of same age has different thinking ability and mental status.
Thus, the answer is falsifiability.