Hi Veronique!
So, 2 nieces going on their first Disney trip?
The 2 parks that would come to the top of my daughter's list (now 7) would be Magic Kingdom at #1, followed by Animal Kingdom at #2.
At Magic Kingdom, the magic begins for your 2 princess nieces. First, I would get them princess tiaras, which you can get for under $20 at one of the Disney shops. I would then take them to Cinderella's Castle for pictures in front of the iconic castle.
Then take them on some of the fun, kid-friendly rides and attractions such as The Magic Carpets of Aladdin, Peter Pan's Flight, etc. If you can fit it in, perhaps a reservation at Bibbidi Bobbidi Boutique (located in Cinderella's Castle), where they can be transformed into little princesses (one of my daughter's favorite experiences at 5). They do have Cinderella's Royal Table restaurant, but that is a little pricey and the food is only OK.
I put Animal Kingdom at #2, as most children that age really love animals. You get a chance to pet the animals at the Affection Section, take them to the Boneyard (a favorite for my children), and of course, can't forget the Kilimanjaro Safari.
To save money, what we've done is buy items off-site from stores that carry official Disney licensed apparel, such as JC Penney or Target. That's a way to cut costs. If you buy it all from Disney Parks, expect to pay those premium prices.
Good luck! Hope your nieces and family have a magical time!