From a young age, children have a nuanced understanding of fairness.
New University of Michigan research indicates that children as young as 5 incorporate market concerns—the idea that what you get is in line with what you give or offer—into their decision making, and increasingly do so with age.
Some people think children are innately selfish—they want to get goodies for themselves. Other people think children are innately altruistic—they care about helping others. Most people think children are both.