I would feel like a horrible writer if I did this but it happens so often that TV Tropes has a page about it: "Generally, if writers want to remove a character from their ensemble, they will either kill that character off or 'put him on a bus' (or both) to explain their absence. Sufferers of Chuck Cunningham Syndrome, on the other hand, simply disappear into limbo. They will often be retconned right out of the story's history, while, of course, everyone still left on-screen will simply carry on as if 'Oceania had always been at war with Eastasia' and 'Greedo had always shot first'. This is sometimes caused by the writers gradually losing interest in the character and, without making a conscious decision to remove them, eventually forget about them entirely. More often, complications behind the scenes drive the decision to remove a character. " http://tvtropes.org/pmwiki/pmwiki.php/Main/ChuckCunninghamSyndrome
Re: missing children, addendum
"Generally, if writers want to remove a character from their ensemble, they will either kill that character off or 'put him on a bus' (or both) to explain their absence. Sufferers of Chuck Cunningham Syndrome, on the other hand, simply disappear into limbo. They will often be retconned right out of the story's history, while, of course, everyone still left on-screen will simply carry on as if 'Oceania had always been at war with Eastasia' and 'Greedo had always shot first'. This is sometimes caused by the writers gradually losing interest in the character and, without making a conscious decision to remove them, eventually forget about them entirely. More often, complications behind the scenes drive the decision to remove a character. "
http://tvtropes.org/pmwiki/pmwiki.php/Main/ChuckCunninghamSyndrome
Luckily fandom has a sense of humour :D