I think the AM was *trying* to imply that in the code snippet, you've left out the parent's new() call in the child's constructor, so the child never gets tied to the db. Of course, s/he could have just said that :)

Maybe you could make the child DESTROY method update its parent object's data instead, so only the parent class is allowed to update the db...I suspect you're always going to have problems having multiple objects writing to a db this way unless you get a nice marshalling system going.
Best o' luck.