In The Little Prince, a sharp contrast is drawn between children and adults. What are three ways that the characters or events of the book show this contrast?

In The Little Prince, adults are not always portrayed in the most flattering light, particularly in comparison to children, of whom the Little Prince himself is the ideal.
Adults are more worried about material possession than children. While children can be greedy, adults make having as much money and as many possessions as possible an unhealthy obsession. We see this with the businessman who believes he owns all the stars in the galaxy, counting them obsessively and becoming blind to their beauty as a result. The Little Prince owns only a rose and a small planet, but he is content with that.
Adults and children have different ideas of reality. The adult can only see what is literally there and, as a result, assumes the surface is all there is to life—neckties, politics, and so forth, as the narrator tells us. However, children are more concerned with matters of the heart and emotional truth. The Little Prince is more concerned with the welfare of a single rose than anything else. The simple love he gets from the rose means more to him than his own life—quite literally, since the narrator is more concerned with bodily survival in the desert than his child companion is. This behavior is not mere childish folly—the Little Prince knows that without love, life is meaningless.
Children are more open-minded than adults. Once again, the adult just accepts everything at face value, while a child always pries as to why things are as they are or why adults insist they must be as they are. The child does not accept "just because" as an answer. We see this over and over again in The Little Prince when the prince encounters different adult figures whose obsessions and miseries seem strange and even needless to him. For example, the King is lonely and sad, but his isolation assures him he is the greatest and most important man on his own little world. For the Little Prince, this is bizarre and incomprehensible, but the King cannot see reality any other way.

Adults lack the imagination of children. This is shown at the beginning of the book, when the pilot shows the adults his picture of the elephant inside of the boa constrictor. The adults uniformly think it is a hat. Only the Little Prince immediately sees what the pilot has depicted.
Adults, unlike children, don't see with the heart. This is illustrated when the pilot is stranded in the desert with the Little Prince. The Little Prince is concerned with matters of the heart: whether his beloved rose will be eaten. The pilot is short with him, concerned about "more important" things like fixing the plane so that they don't die of dehydration. But to a child's eyes, matters of the heart are far more important than practicalities.
Adults are obsessed with owning and counting things, such as the man who owns the stars and spends his days counting them. A child would not care about owning them. Instead a child would appreciate their beauty and wonder.