Season 3 shows us that most of the personnel are living deads, including Sadako from The Ring, so how do they have children?
I have a theory about that: those employees may have been fully alive until said Christmas episode, when Cindy slaughtered everyone in the room. They were then brought back to life, more or less, because the workforce was needed.
The secretaries are undead. The rest of the personnel need not be such. Most of the Flander's employees are actually supervillains, remember. Which would explain why the kids were such terrors.