The House of Arden by Edith Nesbit

Imprisoned in the Tower of London, accused of high treason, and having confessed to a too intimate knowledge of the Gunpowder Plot, Elfrida could not help feeling that it would be nice to be back again in her own time, and at Arden, where, if you left events alone, and didn't interfere with them by any sort of magic mouldiwarpiness, nothing dangerous, romantic or thrilling would ever happen. And yet, when she was there, as you know, she never could let events alone. She and Edred could not be content with that castle and that house which, even as they stood, would have made you and me so perfectly happy. They wanted the treasure, and they -- Elfrida especially -- wanted adventures. Well, now they had got an adventure, both of them. There was no knowing how it would turn out either, and that, after all, is the essence of adventures. Edred was lodged with Lord Arden and several other gentlemen in the White Tower, and Elfrida and Lady Arden were in quite a different part of the building.

And the children were not allowed to meet. This, of course, made it impossible for either of them to try to get back to their own times. For though they sometimes quarrelled, as you know, they were really fond of each other, and most of us would hesitate to leave even a person we were not very fond of alone a prisoner in the Tower in the time of James I and the Gunpowder Plot.