Unknown Child Syndrome
Similar to "Jumping the Shark", Unknown Child Syndrome is a phrase that I have coined to describe the fatal disease that TV shows suffer from when the writers have sunk to the lowest of the low of dramatic storylines. A previously unknown child is added to the cast.
The actor/actress is not to be blamed as they are just a child and probably didn't realize that their character was carrying a disease that would infect the host show and eventually kill it.
The following are the known cases of Unknown Child Syndrome. Any additions are greatly welcomed.