Why is this?
Because children haven't learned how to mask their emotions yet. They haven't discovered the art of manipulating their words and actions to portray what they want other people to think of them. Therefore their emotions are true and genuine - however pleasant or unpleasant it may be at the time.
Dallas Willard has a great quote about this in The Divine Conspiracy:
Interestingly “growing up” is largely a matter of learning to hide our spirit behind our face, eyes and language so that we can evade and manage others to achieve what we want and avoid what we fear. By contrast, the child’s face is constantly an epiphany because it doesn’t yet know how to do this. It cannot manage its face. This is also true of adults in moments of great feeling – which is one reason why feeling is both greatly treasured and greatly feared.
As a father, I'm beginning to understand how gratifying genuine joy is from my sons. Funny how it often takes children to teach the most profound lessons.Those who have attained considerable spiritual stature are frequently noted for their “childlikeness”. What this really means is that they do not use their face and body to hide their spiritual reality. In their body they are genuinely present to those around them. That is a great spiritual attainment or gift.