Not on twitter. No idea who or what. Looked it up. Found an article that explains:
' My Body is Me!, by Rachel Rooney, has been described as representing “an extreme ideology that explicitly targets children” by a fellow children’s author, Clara Vulliamy, creator of the Dotty Detective series.
Rooney, 57, wrote the book to counteract what she described as an “explosion” of titles that promoted the idea that children who felt confused about their gender may be “trapped in the wrong body”.
She said that her book, illustrated by Jessica Ahlberg, taught children to be comfortable with their bodies and featured characters of different races and with disabilities. The book says: “You’re born in your body, you don’t have a spare, so love it, hug it, treat it with care.”
Here's a link to thearticle but it is a year old and doesn't have Philip Pullman :
www.peaktrans.org/rachel-rooneys-childrens-book-my-body-is-me-labelled-anti%E2%80%91trans-extremism-the-times-16-12-19/