The Psammead symbolizes tradition and common sense. It longs for the days when children made sensible wishes—for healthy food—and hints several times that what the five children need is “good tempers, or common sense, or manners” (36). It tells them that they ought to be careful with what they wish for, giving the example of a child who wished he was dead—and was, until sunset.
The children are well aware that their wishes are silly and always go wrong. Cyril points out in Chapter 3 that they are getting in the habit of longing for sunset and wishes that they could ask for something “sensible […] so that [they] should be quite sorry when sunset came” (73). However, they are children, and rambunctious ones at that. They forget and make accidental wishes, or they fail to consider what might happen if they wished that the house were a besieged castle. This tension between their understanding of what they should do and the things they actually do is a major source of conflict in the plot and develops the themes of The Importance of Responsibility When Using Power and
By E. Nesbit