The Enchanted Castle and Five Children and It by Edith Nesbit

The Enchanted Castle (1907) begins when three children stumble upon a mysterious house and discover an invisible princess and a magic ring. At first it all appears to be a great adventure. When the children need an audience for a play they have mounted, they make their own out of old clothes, pillows, and umbrellas. Then things go inexplicably wrong. To the young dramatists' horror, as the curtain falls, there is a ghastly applause. The creatures have come alive—and they prove to be most disagreeable!