The increased capacity for self-regulation that is seen in middle and late childhood is linked to developmental advances in the:

Brain's prefrontal cortex.

Explanation:

Brain may be defined as one of the most important organ of the body that helps in the control and coordination of the body. Three main divisions of the brain are forebrain, midbrain and hindbrain.

The prefrontal cortex may be defined as the part of brain situated at the frontal lobe. This area of the brain is involved in the planning, thinking and the complex behavior of an individual. This area of brain is also involved in the development of the self esteem.

Thus, the answer is brain's prefrontal cortex.  

fluid balance is an aspect of the homeostasis of organisms in which the amount of water in the organism needs to be controlled, via osmoregulation and behavior, such that the concentrations of electrolytes (salts in solution) in the various body fluids are kept within healthy ranges. the core principle of fluid balance is that the amount of water lost from the body must equal the amount of water taken in.