I understand your concern, Andy, when I read the following from Wikipedia:
Pullman is a Distinguished Supporter of the British Humanist Association and an Honorary Associate of the National Secular Society.
The His Dark Materials books have been at the heart of controversy, especially with certain Christian groups. Some, including Peter Hitchens, claim that he actively pursues an anti-Christian agenda.[4] Proponents of this view cite the critical articles he has written regarding C. S. Lewis' series The Chronicles of Narnia (which Pullman denounces as religious propaganda), and the usually negative portrayal of the "Church" in His Dark Materials.
The two series have some resemblance. Both feature children facing adult moral choices, talking animals, religious allegories, parallel worlds, and concern the ultimate fate of those worlds. The first published Narnia book, The Lion, the Witch and the Wardrobe, begins with a young girl hiding in a wardrobe, as does the first His Dark Materials book, Northern Lights (published as The Golden Compass in the US). Pullman's hatred of the Narnia books is shared by some liberal commentators such as Polly Toynbee.[5]
Some, including Hitchens again, have seen the His Dark Materials series as a direct rebuttal of C. S. Lewis's series.[6] Pullman has also criticised the way Lewis excludes the character Susan from the final 'heaven' scenes in The Last Battle, saying she is rejected for her growing worldliness.[7] Lewis devotees argue that Pullman has read too deeply into this; Lewis made no statement about Susan's ultimate destiny, and never excluded the possibility of her rejoining her friends in heaven later, as they are dead and she is still alive.
However, Pullman has found support from other Christians, most notably Rowan Williams, the Archbishop of Canterbury. These groups and individuals point out that Pullman's attacks are focused on the constraints of dogmatism and the use of religion to oppress, not on Christianity itself. Dr. Williams has gone so far as to propose that His Dark Materials be taught as part of religious education in schools. Moreover, even authors of works dedicated to critical appraisals of religious themes in his writing have described Pullman as a friendly and generous debating partner[8].
Source
Although raised an Anglican, I am not exactly enamoured of Rowan Williams therefore do not consider his counsel necessarily to be that wise - not in other matters, therefore maybe not in this one either.
I do remember hearing somewhere of a father who allowed his children to read quite widely but with one proviso - that they discussed the book fully with him when they had done so. The father then used the book, and the thoughts that the child had about it, as a teaching opportunity to explore the ideas and develop them more critically. I wonder, Andy, if this might be an option to consider? Your child, if developing a Christian world view, might learn to critique the material from such a perspective and have some interesting things to present in a review of it in class work.
Unfortunately we cannot wrap our kids in cottonwool and keep them from being in the world. They do need to learn about the world, and how to handle it as Christians. It is a challenge, but it could be one that brings benefits through strengthening them to deal with other issues further along the path.
My suggestion is perhaps a compromise, but maybe not a bad one.
School problems
) then I would probably complain.