Hi!
What a festive idea
Your daughters will have a blast!
My DD is 8 so I am a little unsure of what a 12/13-year-old would like best but I would plan on at least one character meal - preferably Cinderella's Royal Table at the Magic Kingdom or Princess Storybook Breakfast at Epcot... Have they ever been to one of these? Even though they're probably getting "too cool" for the Princesses, I think they would get a kick out of it!!
Where are you thinking of staying? March will be Peak Season so prices will be steep, but there is currently a Winter Resort Offer and I'm betting there will be a Spring Resort offer as well. Also, do you have an Annual Pass? If made available, this will make you eligible for AP rates which can be up to 40% off rack rate.
Thinking back to when I was that age, I would consider splurging on the nicest hotel you can afford - is GF in your budget? I think they would be wowed!!
On the other hand, I'll bet they'll love MGM Studios, so perhaps an Epcot resort instead (we LOVED BWI in July!
)
Sorry I'm rambling - and not even much help! but I'll be interested to see what others say
Good Luck!