Liz:
There's no better place to indulge your daughter's penchant for princesses than at one of the Princess Dining experiences at Disney. If you want to kill two birds with one stone (although, that may not be the best way for me to phrase that...)--If you want to see both princesses in one palace, your best bet: the Princess Storybook Dining at Epcot's
Akershus restaurant. While dining here, half a dozen of Disney's princesses greet guests and give you time for pictures, autographs and interaction---without long waits in the parks or intentional planning. Both Jasmine and Ariel typically will be there to meet your daughter.
Outside of this dining experience, these princesses will not be in the same location. Jasmine now appears sometimes at Cinderella's Royal Table for Breakfast and Lunch along with some sightings in Adventureland in the Magic Kingdom. You can also find her near Morocco in Epcot with Prince Ali.
With construction going on to expand Magic Kingdom's Toontown, the ever-popular Ariel's Grotto has now closed, but she still makes appearances in the park. I would suggest getting the Times Guide in the train station as you enter Magic Kingdom to see where she'll be. Disney's Hollywood Studios also hosts her Voyage Under the Sea, so look at its Times Guide, too.
I also recommend that if you don't see your favorite princess on the Times Guide to ask a Cast Member in Guest Services to call the Character Hotline for the most up-to-date information!
Wishing y'all a magical--and memorable vacation!
Whitney