There are some things working against them. First, kids' bodies aren't built to keep up an even pace over a long stretch, so the energy cost is more demanding than for an adult.
In other words, they expend more energy than we do just to maintain pace. Also, children's hearts are smaller, so kids get less oxygen and energy to their working muscles during sustained running.
What's more, training for this distance may quickly become daunting for the child. It's hard for kids to stay focused, especially during long training runs.
Children are the world's greatest interval trainers, but they don't normally take to long-duration, slow-moving activities. If you can do things to make the distance training fun (kicking a ball or somehow make a game out of it), this will help.
Other than these adjustments for age, a child's training should be the same as an adult's. Perhaps most importantly, they're going to have to get in a long run every two weeks or so and should be able to cover 10 miles comfortably two weeks before race day. Also: