Just wanted to add one more thing....I just looked over the list of what's included in the Platinum package.
There is one and only one thing in that whole package that's exclusively for package guests....the special viewing areas for Wishes and Illuminations. You can get EVERYTHING else without buying the package.
The Platinum package is a way to pre-pay for EVERY activity that's offered at WDW. If you stayed for a month, you couldn't do it all. But, if you get that package, you're going to be paying for it all whether you do it or not. And, with two very small children, you'd be excluded from many of the activities. I hate to be blunt, but you might as well pile the money up and set fire to it.
Here's my best suggestions for you:
1. Book a concierge room at GF, with an MK view. The GF is the most princess-y resort at WDW. The concierge staff will help you with all of your planning needs.
2. Pick up a copy of Birnbaum's Official Guide to WDW at your local bookstore, or
Amazon. You'll see descriptions of everything that you can do at WDW, and you'll be able to tell what's appropriate for your kids.
3. Start a thread on the Theme Park boards with a title like "Dream Trip for my Princesses". Ask for suggestions on activities that folks would include in a perfect trip for two princesses. You'll get tons and tons of great ideas for activities that will give your girls the most fabulous vacation they could ever have.
I know you want to pull out the stops and give your girls the vacation of a lifetime, but the Platinum plan is not the way to go. It's not meant for families with 4 and 6 year old children. You're thinking of this as a once-in-a-lifetime trip, but if you drop the Platinum plan, I'd bet that you'd have enough money left over for a second trip.