The correct answer would be, Conserve.
You actually gave each of them the same amount, but your niece and nephew lack the mental ability to Conserve.
Explanation:
Mental ability to conserve is called as Conservation. It is a logical thinking process. This logical thinking allows a person to determine that a certain quantity of anything will remain the same despite of the separate containers which may have different shape, size, etc.
So when your niece and nephew were unable to understand that the quantity of milk given to them is same despite of the different shape and size of the container, they actually lack the ability to conserve.
Learn more about conservation at:
https://brainly.com/question/11492624
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