I wonder who they would be .... children of the light and shadow. If it is at all possible for Sylvanas to have children because she is undead. But on the other hand she looks completely different than other undead creatures. Rather like a blue elf with red eyes and smooth skin, not like ghulls, skeletons and other walking corpses. In Twilight for instance Edward was also undead and despite that fact he was able to have child with Bella (Renesmee). It is interesting because children who could wield power of light and darkness simultaneously .... must be extremely powerful.