Books like The children's country by Katharine Burdekin

This is a fantasy in which two children, foster-siblings Donald and Carol, invoke a spell on St John's Eve and are transported to Fairyland from their home in Scotland. A Border-Keeper sets them two tasks to complete satisfactorily before they are allowed to enter. They are given a Guide, Gilly, who, like all of the inhabitants of the Children's Country, is neither a boy nor a girl, just a Child. With other Children, Donald and Carol journey to the People's Country, where they separate for a time, Donald going off with Gilly in search of new adventures. But Gilly vanishes, and Donald strays into a Witch Valley, where the Witch, who is as beautiful as she is deadly, is slowly killing him taking his life-force with her kisses. Carol searches for Donald for months and finally finds him defies the witch, and carries him out of the Witch Valley. After he recovers, they are sent home by the Border-Keeper, and find themselves back in Scotland as if no time at all had elapsed. The book is often mentioned as a non-sexist work, but it is so much more than that. Its influences clearly range from traditional fairy tales through Greek mythology and medieval romances, but Burdekin transcends her sources by creating two modern children (the book was published in 1929) whose journey to a different universe transforms them in ways that not only question gender and the idea that biology-is-destiny but that also bring up issues of politics, power, freedom, and service.

First publish date: 1929