The story was about a group of girls, I think 5, that all lived by a tree house or some sort of well. They all got coins in their mailbox and by using them they went to a different world. I don't quite remeber the rest of the book but I do remember one of the girls turing into a mermaid or somtheing simlar. . The book was at a 2nd-grade reading level and it was a part of a series. Any ideas as to what it is would be greatly appreciated!

The story was about a group of girls, I think 5, that all lived by a tree house or some sort of well. They all got coins in their mailbox and by using them they went to a different world. I don't quite remember the rest of the book, but I do remember one of the girls turning into a mermaid(?)

The book was at a 2nd-grade reading level and it was a part of a series.