Tennis demands quick stopping, starting and changes in direction.
While agility drills can help with this, playing basketball and soccer are great.
To play defense the player has to learn to keep the center of gravity low to zig when their opponent zigs, and zag when their opponent zags.
USTA agility drills: http://assets.usta.com/assets/1/USTA...oc_437_269.pdf
The spider drills and horizontal repeaters are favorites as they mimic the extent of movement on court for those unlikely to rush the net.
Swing the racquet before each change in direction.
Doing agility exercises can be tricky because it needs to stay fun for a ten year old.
Some will like to beat their best time to go through 2-3 sets of these exercises - many will feel like they are being over worked.