I read this book about 10 years ago in elementary. It was the first book in a series. It starts off with kids playing by a tree and then one of them goes down a well to find a money printer. He also finds a nugget of gold down there. There is also a detective that comes in later in the story.
Is it The Island of Adventure (1944)?
This is the first book in the series. Some children explore an island near where they are staying, and discover a counterfeiting operation in some copper mines. The tunnels connect to the well at their house.
Excerpts from Anita Bensoussane's review via The Enid Blyton Society:
[T]he children visit the Isle of Gloom, where the rocks contain deposits of copper (Blyton initially described their colour as red, but in later editions this was corrected to green.) On the island they see men apparently working the old copper mines, and assume that Bill must be in league with them. Needless to say, the children are wrong. They eventually find out that Bill is a member of the police force who has been detailed to keep an eye on Jo-Jo. It is Jo-Jo who is in league with the men on the island, who are not mining copper but using the old mines as a place in which to make counterfeit money.
Because a secret passage links the mines with the well at Craggy-Tops, the well-water turns salt, meaning that Aunt Polly and Uncle Jocelyn will have to leave the old house.
See also Kids find money printing operation in goldmines on Goodreads.