This 2014 human study framed many of the other studies that relate to children’s brain development:

“The researchers found instead that the brain maxes out its glucose use at age 5. At age 4 the brain consumes glucose at a rate comparable to 66 percent of the body’s resting metabolic rate (43 percent of the body’s total energy expenditure).

To compensate for these heavy energy demands of our big brains, children grow more slowly and are less physically active during this age range.”

http://www.pnas.org/content/111/36/13010.full “Metabolic costs and evolutionary implications of human brain development”