Yes, you can walk but it is a lengthy one - especially for a tired youngster or oldster.
If this is your child's first time in the park, I'd suggest you enter through the Main Gate rather than the monorail. Let them experience their first time entering the park by seeing the Mickey floral arrangement and the train station then gaze down Main Street with the castle at the the end. In fact, have them close their eyes then guide them to the top of Main Street and then tell them to open their eyes so they can see the castle. It's a magical experience and will bring a tear to your eyes when you see their expression.
Then go to City Hall and get a First Time Visit pin for them to wear. They'll get lots of special attention from the cast members.
On your subsequent days, enter on the monorail which takes you above the main entrance and into Tomorrowland right near Autopia and the Submarine Grotto. It's a lot of fun and a great birds-eye view of the park.