The description of the Crossfit Kids program states that this is not just a scaled version of crossfit, but is actually a specifically designed program to meet the special needs of kids. That holds true for several areas, including running and endurance training. Crossfit teaches that training with short, interval runs will help your ability to complete longer aerobic runs. For kids, this is even more appropriate. Children have an ability to recover from intensity at a faster rate, and they have a naturally higher aerobic threshold (longer training is easier for them). This means that their training should focus more on intensity and speed. Since they can already manage long distances, and their system is ready to recover from some intense intervals, training time is best spent on speed and technique.
If a child wants to work on running a 5K or 10K, they will become much faster if they focus on various versions of sprints and short distance runs. Workouts can include 200-400m sprint repeats, time trials, or runs alternated with body weight movements. Time spent on form is also important. Children are already inclined to run fast, but their form will break down over time. With practice on body position and efficiency of movement, their energy will all be directed at forward movement. Training for a 10K run with lots of long, slow runs will only teach them to run slow. Once established, this is a hard pattern to break. Instead the workouts should incorporate adequate rest between shorter intervals. Because children already recover quickly, they will be ready for more intervals sooner than adults. One of the side effects of this type of training training is the inherent fun that comes with sprinting. Kids will enjoy the intervals with rest much more than a long drudge through 30 minutes of a 3-4 mile run.
For more on endurance training with kids, check out this website: