The perception's common that small children are extremely immature and entitled. They cry whenever they don't get everything they want, even when it's not something they're entitled to! What selfish behavior! Or is it?
As adults, we have a biased perspective because we know about things they don't. We've seen what's in the world. We work, see horrible atrocities going on all over the place, and whatnot. But children don't know that those things exist. They've never seen anything but play (well, and abuse). They legitimately don't know what it means to have a job, or to have to work for food. From their perspective, there is nothing else in life than getting things they want. It's literally the most important thing in the world by elimination. So why not cry about it?
The real maturity is to recognize when another person doesn't see what you see, and to judge them from their own perspective.