Hi Heather,
How positively adorable that your little one is so enamored with Prince Eric and Ariel! I have a few ideas that I think she will love when you visit Walt Disney World.
A new character dining experience called
Bon Voyage Breakfast recently debuted at Trattoria al Forno restaurant on Disney's BoardWalk. This breakfast features Rapunzel and Flynn Rider, as well as Ariel and Prince Eric! I would suggest that you make a reservation for this as this is likely the only place she will be able to meet Prince Eric. I also want to let you know that character appearances are subject to change, so it might be prudent not to tell her that he will be there and let it be a surprise! You mention that you are heading to Walt Disney World in a couple of weeks, so it might be tricky to get a reservation this close to the date. I suggest that you be flexible with the date and time of your reservation.
At Magic Kingdom, there are many opportunities for Little Mermaid experiences. You can see both Ariel and Prince Eric in audio-animatronic form on Under the Sea - Journey of the Little Mermaid, a ride-through of the story of the movie. You can also see Ariel in the Festival of Fantasy Parade at Magic Kingdom, where she leads a colorful section of the parade. Finally, you can
meet Ariel at her Grotto. At Disney's Hollywood Studios, you can also see a stage retelling of the Little Mermaid in
Voyage of the Little Mermaid.
I hope that these thoughts are helpful for you. Please let us know if you have any further questions, Heather!