Simon & Schuster, which will publish the first book in the series in spring 2022, said the deal was believed to be record-breaking for a debut children’s author.
Steadman said she had wanted to be an author “for as long as I can remember”, but was “just so overworked”.
“I just wrote some names of unicorns, and then I left it alone,” she said.
“At the moment I’m not even sure I’ve really processed [the size of the book deal] at all.