One way that a child stands out: emotions don't "stick" with them more than 15 minutes. It is as if their personas are teflon coated so emotions slide off. Sure the normal interactions of the day will bring anger, frustration, sadness, joy, etc. But what's remarkable is that these dispositions move along, like clouds moseying across a blue sky to make an appointment beyond the horizon.
Adults, well, all too often emotions stick to us one at a time, the way barnacles latch to the hull of a ship. Think of how someone's comments or actions make you stew inside. For how long? And why so long? Perhaps it is the scale or intensity of what we experience.
But is it something else? Perhaps that when we are young we live in the moment, as veritable kunduns; perplexingly, as we "grow up," we teach ourselves too well how to live in the past and the future. Hard to know why or how this evolves in us, but I have seen the stark difference, now that we have two young boys.