The Swing in the Summerhouse — Jane Langton

When Prince Krishna is called away, he leaves behind some strict instructions for the Hall children about one of the openings in the mysterious summerhouse: Keep Out! As Eddy and Eleanor swing through each of the other openings, they refuse to break the rule, even as their temptation grows. But when Oliver and little Georgie disappear through the forbidden archway, Eleanor and Eddy know that they must either break their promise or risk never seeing their friends again.