absolutely shocking its heartbreaking to hear that the child is asking about why is it always dark,
with regards to your question im no expert at all but as far as i know there is currently no treatment if its the case of the optic nerve, with stem cell technology and nanotechnology, there may be hope in the future.
The latest i heard on tv was that one can be partially restored with the vision, which is good but the other eve is completely damaged and a doctor on tv said that there are over a million nerves so it complex to fix, but there are still new remedies that may be available in future.