Play games after race days or at the end of practices to build fitness, general athletic skills, and enjoyment of running.
Have fun with all practice activities, but also include time specifically for games. Even though at this age runners should not be training so hard that they need a physical break, having game days after races or hard practices is a nice way to reward your team's effort. Games are also great for the last 10-20 minutes of practice before cooling down to keep your team moving when they may otherwise begin to get sluggish, which will improve their overall fitness.
Games also include elements of speed, agility, coordination and/or power, thus building general athletic skill. Equally important, games build team spirit and camaraderie and keep athletes having fun and looking forward to practice. The more they practice, the better they get. When kids see their improvement, they enjoy it even more, so the cycle continues.