Welcome aboard, Amber! I just returned from a 6-night sailing on the Disney Magic out of Galveston last week and can tell you that you are in for a great cruise!
My daughter loves Merida as well, and I wish I had better news for you, but currently Merida is not appearing on Disney Cruise Line. I hope that she will in the future, but for now, your best bet is to meet her at the Magic Kingdom at Walt Disney World.
When you get to the park, head to the right side of Cinderella Castle, and there you will find her in
Fairytale Garden. It's a great spot to meet her, and one of my favorite character greeting locations, as your daughter will have some fun things to do while she waits in the queue. My kids loved learning to shoot a bow and arrow just like Merida, as well as coloring some Brave pictures. Merida's three little cub brothers are sitting there on the cart behind her and might just pop up to greet your daughter!
The line to meet her can get long, so I suggest going early. It's usually open around 9, but check your park map for that day's specific greeting times when you arrive.
In the meantime, you can read more about
Merida's Disney meet and greet in this article from the Disney Parks Blog.
Have a wonderful cruise and day at the Walt Disney World resort, Amber!