As a person who worked many, MANY years doing all sorts of customer service related jobs (on the phone), I can't just default to sympathy for the OP.
I'm not saying the CM with whom she spoke handled this in the most mature, professional manner. But it sounds like the OP hit some of the nerves that people who working in customer service develope, so while the CM was wrong, it's not at all surprising.
I realize that most people think it's perfectly acceptable to treat the person making $8 an hour on the phone like they are personally responsible for all the sins & frustrations of the company they're working for, and like they personally are benefitting from whatever raw deal the caller thinks they're getting (or by extension, will be personally hurt by any threatened withdrawl of business).
I know all the OP wanted was for the CM to give her exactly what she wanted and apologize for upsetting her day. But sometimes they can't do that. If *I* as a customer services rep can not (because of company policy, system requirements, or a thunderstorm that knocked out my system) give you exactly what you want, how you want it & when you want it, that does not mean *I* am evil, *I* am incompitent, or that *YOU* have the right to abuse me.
So - was this person's actual sin that she said that they couldn't put the OP, spouse & child at the same table? Or that she said she couldn't change the printed confirmation to say that - and the OP didn't want to just hear that they'd be seated together anyway, regardless of what it said? Or maybe whatever the problem was with the system, the first CM didn't know why it happened or how to fix it - and wasn't getting any support to know when it would be fixed or how to fix it?
Would that have been Disney's fault? You bet. But that CM making $8 an hour didn't deserved to be abused over it. And I'm sorry - but with all due respect, after 15 years doing CS - there's no way the caller went from sweetness & light to "we're cancelling our entire trip over this!" without any negative words in between. That doesn't make the CM right - but really, what do you expect?