Run up a hill.
Drag a tire or sled.
short distances, 20 yds at most. not a lot of reps, 3-6.
Jumps in place. Importance here is that the less time they spend on the ground, the faster they are.
Must develop hamstrings, glutes, low back and calf muscles. Kids can do bodyweight squats, KB squats and swings, lunges, toe raises, back raises, band good mornings.