I have planned many last minute trips with my family of 5! And every time we have a blast. Yes, you don't necessarily get the resort/room category you want, or don't always get the ADR's you want, but you can definitely make the best out of it.
My advice is to check back often for ADR's that you are really wanting- we almost always get everything we want due to cancellations a few days before. With the new credit card guarantee for some restaurants, you will find people canceling around 24-48 hours before. So keep checking often, especially right before/during your trip if possible.
And we have been booking room only lately on our last minute trips. There is no penalty for changing dates as long as you are 5 days or longer from your trip. And the room balance isn't due until arrival, but you can pay in advance if you prefer (which we do). We usually purchase our tickets through the Disney World website or when we get the the resort.
Let us know if you have any more questions, and I hope that information helps!!!