This is mainly directed at soap operas but I'm sure there are examples in prime time tv as well.

Children go away to boarding school or just fall off the planet without ever being mentioned - even by their parents.

I can't remember the last time The Young And The Restless' Victoria Newman mentioned her son Reed. I don't even know where he went. Same with Lily Winters' twin children who left town ages ago and are never mentioned.

What are some other examples?