Read "The Enchanted Castle" by Edith Nesbit online for free on Textopian. Full text with search, annotations, highlights, and AI-powered reading aids.
The difficulty was not only that Gerald had got the ring on and couldn't get it off, and was therefore invisible, but that Mabel, who had been invisible and therefore possible to be smuggled into the house, was now plain to be seen and impossible for smuggling purposes.
The children would have not only to account for the apparent absence of one of themselves, but for the obvious presence of a perfect stranger.