I would argue that the book is an endorsement of transgender ideology, and as such they are breaking the law on the requirement to be politically impartial, unless they also provide the children with age suitable material that is against the ideology.
You could suggest "Johnny the Walrus" by Matt Walsh.
So they need to teach both sides or they are breaking the law.
I would be very interested in whether they are willing to read a Matt Walsh book to the kids. Maybe they would see that as some sort of hate crime, I don't know.
You could also mention that GC beliefs are protected under the Equality Act, so what excuse would they have for not teaching them with other perspectives?
(Matt Walsh isn't GC, but conservative.)
A different option, is that the book is teaching that a young child already knows their "true self". But many children may recover from gender dysphoria, so this kind of messaging is medically wrong and potentially harmful.
Or we are still battling over these new issues as a society.
When we haven't worked this stuff out yet in the adult world, maybe we shouldn't be targeting children with propaganda in a culture war that they can't possibly understand.