A primary school teacher, Harrison, is observing children playing with a model farmyard. He notices that 5-year-old Toby expects other children to see the farmyard from the exact same perspective as him, even when they are sitting on the opposite side of the table. In contrast, 9-year-old Grace can easily describe how the farmyard looks to her friend sitting opposite.

With reference to Harrison's observations, outline the key research by Piaget and Inhelder (1956) and explain what it tells us about cognitive development in children.