The Miraculous Sweetmakers: The Frost Fair by Natasha Hastings

A spellbinding and unforgettable tale of deep friendship, glittering magic and enchanting mystery, perfect for fans of Tamzin Merchant, Abi Elphinstone and Anna James. The Great Frost of 1683 has London in its icy grip. Thomasina and her best friend Anne sell sweets on the frozen Thames, amid rumours of the magical Frost Fair that awakens there at night.

They say if you can find the fair, Father Winter himself will grant you any wish. And Thomasina has an impossible wish: the return of her twin brother, Arthur. But once they discover Father Winter's kingdom, Thomasina and Anne quickly realize the Frost Fair is not what it seems and that some wishes never come for free