I just read through this thread and I am so sorry that all of this is happening! I understand your not wanting to play hardball with her so close to your big day, but the whole thing is absolutely ridiculous! I would honestly go through every email and piece of paper you have from them and see where it says that 30 vs. 50 people will cost more. If it doesn't, and that issue has NEVER been brought up before, they cannot change the price at the last minute. I got married at a small location, and there was absolutely something in the contract about price based on number of guests.
Also, if your prior contract has everything broken down with decorations, etc included, I would point it out to her. If she wants to charge you full price drinks for the kids, I would suck that up... its probably the least of the issues. Its money-grubbing and crazy to think that a 7 year old will cost as much in drinks as an adult drinking alcohol, but whatever.
She obviously has decided that she didn't like the prices her niece was quoting you. The thing is, whether she likes it or not, that is who she delegated to direct weddings. I would point that out to her... and say that you NEVER would have chosen the location if you had known it would cost that much, and that you have everything in writing for what you were quoted.
I might also direct her towards BDW now, and show her that destination brides are in a way united, and certainly have influence over her potential future clientele. A good review goes a very long way around here, and a bad review, even a mediocre review, can sway someone away from a place.
Are you staying there as well as having the wedding there?