Five-year-olds who were surprised to discover that a band-aids box contained pencils were able to anticipate their friend's false belief about the contents of the box. this best illustrates that the children had developed a

theory of mind

Explanation:

Based on the information provided within the question it can be said that this best illustrates that the children had developed a theory of mind. This theory refers to an individuals ability to attribute their own emotions, desires, knowledge or beliefs to other individuals as well as themselves. Which in this scenario since the Five-year-old's had already experienced the surprise of finding the pencils inside the Band-Aid box then they knew how their friends were going to feel when they opened the Band-Aid-Box.