Hi Rosil,
What a special day you are planning, a visit to Magic Kingdom for a 5th birthday! Let me offer you some thoughts on making this day magical.
First, do be sure to stop by Guest Relations and pick up a Happy Birthday button. These are available free of charge, and although no extra magic is guaranteed, you never know when a little extra pixie dust will be sprinkled on a birthday guest! I will never forget the look in my daughter's eyes when a cast member at The Plaza Restaurant surprised her with a huge cupcake as a birthday treat after her lunch! The cast member had noticed her birthday button and made this magical moment for her.
Since you mention that your little one loves trains, you could start your visit with a ride on the Walt Disney World Railroad. This attraction circles Magic Kingdom starting on Main Street U.S.A., and makes stops in Frontierland and Fantasyland. If the birthday boy is at least 40 inches tall and ready for a mild thrill, you could debark the Walt Disney World Railroad in Frontierland for a spin on Big Thunder Mountain Railroad.
As for seeing Mickey Mouse, Magic Kingdom is the place to be! You can see him in so many places, including the Mickey's Royal Friendship Fare show on the Castle Forecourt Stage as well as in the Move It! Shake It! Dance & Play It! Street Party. Check the
Magic Kingdom entertainment listing for showtimes of these and other experiences with Mickey Mouse so that you can plan your day. I would save the best for last:
meeting Mickey Mouse at Town Square Theater. I suggest that you make a Disney FastPass+ selection for this as Mickey Mouse is very popular!
I hope that these thoughts are helpful for you, Rosil. Please do let us know if you have any further questions!