The question posed here and especially the parent's comment about night light causing cancer cannot rely on the publication as hypothetical basis. The article merely states that the lack of melatonin conflicts with the effectiveness of the drug tamoxifen.
Indeed, so let me word that slightly better. If I had a child with leukemia and he or she is prescribed tamoxifen, then should I avoid a night light?
I'm not a doctor, but I would hazard a guess that a child with leukemia would not be prescribed tamoxifen.