汉语refers to the mandarin spoken language. 中文is the written language. Therefore it should be 他们不会读中文书
The English sentence could mean books from China, not just books written in simplified Chinese.
Why not 她们不看汉语书？Surely easy to add?
Just mark it as "My answer is correct" and they will add it as an acceptable response. I have done that with numerous answers and they email me to let me know they have added my translation as a correct option.