The Treasure Hunters by Enid Blyton

The Greyling's Treasure has been lost for many years, but Jeffrey, Susan, and John Greyling find an old map showing where the treasure is hidden while staying with their grandparents. Of course, they decide to find the treasure before their grandparents sell the family home to Mr Potts. However, Mr Potts—whom the children have nicknamed Mr. Pots of Money—is also interested in the treasure, so it becomes a race to see who will find it first...