The most fundamental skill to learn is body awareness, the internal sense of knowing where one's body parts are and how they are moving without looking at them. As kids become aware of their bodies, they will begin to gain control of their movements, and run with greater confidence and ease.
Coordination is the skill of sequencing and timing muscle contractions for efficient movement. The more your kids use and test their coordination, the more coordinated they will become.
The next skill is balance. Good balance leads to joint and posture stability.
Finally, agility is the skill that enables one to change direction quickly. Balance and agility are both important for creating a more stable and responsive athlete.
Your team should play with these skills regularly in games and fun form activities like those in Running Start's Coaching Videos.
Other general athletic skills such as speed, strength, and endurance will begin to develop naturally, but should not be a focus until kids move into middle and high school.