We're going down for a family reunion, and will have 2 days (and very little money) to spend at WDW. We go every year or so, and usually only spend a few days (always in conjunction with visits to relatives to Florida.) We've figured out lots of ways to save.
First of all, check out the discount codes at
http://www.mousesavers.com. We were able to get a room at All Star Music for $55 with a discount code. You can only have 4 in a room there, though, so with your group you'd have to get 2 rooms. (We're in the same boat. I'm going with my mom and my 3 kids, 11, 9 and 3.) You can get 5 in a room at Port Orleans. They're booked for when we're going or we'd stay there.
You can also get 5 in a room at some of the offsite places, but I've never stayed at any of them so I hesitate to recommend them.
If you're staying on site you can get Unlimited Magic Passes, which get you into all the parks. We don't have enough time or money for that this year, so we're spending one day in the Magic Kingdom and one day riding monorails, boats, etc. to the differerent resort areas. For your girls, it would be great if you could get multi-day passes so you can go to all the parks. I'd really suggest getting a copy of Birnbaum's Walt Disney World By Kids for Kids. Let them go through it and figure out what their highest priorties are. Your 3yo will definitely have the most fun at Magic Kingdom (MK).