Five Children and It by Edith Nesbit

Of course you, who see above that this is the eleventh (and last) chapter, know very well that the day of which this chapter tells must be the last on which Cyril, Anthea, Robert, and Jane will have a chance of getting anything out of the Psammead, or Sand-fairy.

But the children themselves did not know this. They were full of rosy visions, and, whereas on other days they had often found it extremely difficult to think of anything really nice to wish for, their brains were now full of the most beautiful and sensible ideas. "This," as Jane remarked afterwards, "is always the way. " Everyone was up extra early that morning, and these plans were hopefully discussed in the garden before breakfast. The old idea of one hundred pounds in modern florins was still first favourite, but there were others that ran it close -- the chief of these being the "pony each" idea. This had a great advantage. You could wish for a pony each during the morning, ride it all day, have it vanish at sunset, and wish it back again next day. Which would be an economy of litter and stabling. But at breakfast two things happened. First, there was a letter from mother. Granny was better, and mother and father hoped to be home that very afternoon. A cheer arose.