Five Children and It (version 4) by NESBIT, E.

When four children (and their baby brother makes five) manage to uncover the long-dormant Psammead (in plain English, then, Sand-Fairy) in a gravel pit, they have the chance of one wish a day that lasts until sunset. But you should be careful what you wish for – as Cyril, Anthea, Robert and Jane find out! (Summary by James Pyle)
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