What happened to Virginia Walton on The Waltons?

**In one of the feature-length movies, we heard that Virginia had died, but were not told how it happened. She would have been in her mid-teens had she been alive.

Charles Benjamin Walton was born in the last series of the show. He isn't mentioned at all in the aformentioned movie, and Cindy is in despair at her childlessness. She minds Erin's three when Erin goes to work. Goodness knows what happened to poor Charles. He can't have died, or Cindy would have been crying about BOTH her children being dead, not just one of them. He would have been about ten years old, had he not just vapourised.

Big plot hole. We'll never know.**