Hi again Lilian! I'm so happy to answer another question for you here at the Moms Panel.
Wishes is such a beautiful and amazing display of fireworks and music surrounding the centerpiece of the Magic Kingdom - Cinderella Castle, I get misty-eyed every time I see it.
I have two areas of the park to focus on to be sure your 5 year-old has a wonderful time. The first is to find a spot anywhere at the north end of Main Street, USA up to the Hub where the Partners statue is. This will ensure you have a clear view of the front of the Castle for Wishes. However, this is also the most popular area to see the show, so you will not be alone. You will most likely have to lift up your 5 year-old so they can see over people and you'll have to stake your spot out very early. Another spot is along the rose garden to the right of the Castle, down to the former swan boat landing area. We actually watched Wishes there in December and had a great view. This front right side of the castle is also a great place to be able to see Tinker Bell's flight as she sets off the fireworks.
My second area is the Main Street Station of the Walt Disney World Railroad. If you are viewing Wishes as the last thing you do before you leave the parks, and you have a very tired 5 year-old on your hands, this will be the best place to make the easiest exit when Wishes ends. However, being that far away from the Castle, you will miss being up close to some of the amazing effects. I just suggest it if fighting the crowds exiting the park is something you want to avoid.
The good news is as long as you can look up into the sky and hear the music, you will most likely enjoy Wishes. These spots will just ensure you either see as much as possible, or can exit easily after the show. I hope this helps you plan and that you have a magical vacation!