Five Children and It (version 4) - E. Nesbit

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)