Two beakers contain what are clearly identical amounts of a liquid. however, a child suddenly judges them as holding different amounts of liquid after the liquid in one beaker is poured into a shorter, wider beaker. the child apparently lacks a concept of your

The answer to this question would be: concept of conservation. 

The concept of conservation will make someone be able to tell whether a quantity is same even if put into a different container. Children will start to develop the concept in age 2 but the concept will be able to use it correctly at age 7-11.