Laurie,
Yes, you'll be eligible for any discounts that are released after your initial booking. In fact, your package can reflect further savings right up until your final payment is made. You'll want to watch your package terms for restrictions, however these usually don't kick in until 45 days prior to your trip.
If you book on your own via phone or online, Disney will not automatically change your reservation to reflect new discounts. You would need to do the research on your own and then call to request the new discount. Usually, all it takes is asking the Cast Member "can you see if code ABC is available for my reservation?". Each
special offer will have a booking code attached to it. The current Kids Stay and Play Free offer has a code of GW8. If you book online, you'll just select the offer that you want to price out.
If you want to be informed of all the latest discounts, I highly recommend following www.mousesavers.com and signing up for the monthly newsletter.
If you want some help with finding the best discounts, utilize the complimentary services of an Authorized Disney Vacation Planner. I highly recommend www.enchanteddestinations.net for you. They are based in Pennsylvania!
I've also had great service with www.dreamsunlimitedtravel.com. The best part of using one of these specialized travel agencies is that they WILL automatically get the best discounts for your reservation. Rest assured that when a new discount is released, they will be on the phone that morning changing your reservation to insure the savings for your family!
Traveling to Walt Disney World with a grand gathering of 17 guests is certain to be a memorable vacation! It will be even more magical if you know you are getting the best rates possible!