Jean Piaget was a psychologist that focused on the cognitive behavior of children. He created a lot of experiments and tests that involved you children examining and presenting whatever claim they found. Piaget was able to conclude that children are able to use two processes of thinking. One being assimilation and the other accommodation.
Assimilation is the process of absorbing new experiences into current knowledge of structures. The accommodation aspect is the process of altering a belief to make it more compatible with experience
In this experiment above it shows that kids are unable to follow the process of the water changing levels. Even though the girl was the one who poured the water into the cup. when she was asked to compare the two she said there was different amounts. This is because they are not able to construct the fact that the two cups have different volumes.
I have a 6 year old brother who is currently in 1st grade i did the coin test on him. I first lined up two rows of eight coins and asked him if they had the same in each row. He responded with yes they do. So i set them at an angle and re asked the question. His first response was different but after I gave him the "look again" look he looked and he counted the coins instead of just looking. So I believe that children if given enough time children would be able to break the illusion and actually see what is happening.