Without question, sprinting (the right way) is one of...if not THE greatest way for your junior squash-player to develop their overall fitness for playing their best squash. ALL good amateur and/or professional squash-players do them. Not only will sprinting help them with their aerobic and anaerobic fitness, but sprinting will help their quickness and power, which is so important in not only getting TO the squash-ball, but also increase their ability to recover (change direction) back to the middle of the court AFTER playing the ball. When we say “the right way,” we mean that each phase of doing sprints should be paid attention to...of which there are about seven phases. 

1) When sprinting from one end of one-minute, eight-stride runs, (in the yard or street) do not have them push-off with the same leg at each ending sprint. They should alternate, first with the right leg, next with the left leg, etc. This will  help them to keep balanced and be strong on both sides. 

2) Do not have them push-off a wall with their hand at each end of the sprint. They may touch it, but don't have them push off. Have them use their forward (left or right) leg and quadricep muscles (upper thighs) to change direction. 

3) Have them sprint in a straight line, not “looping” around at each end of the sprint. Have them run in a straight-run line back and and forth. 

4) Be sure to NOT have them hold their breath, but DO breath OUT when pushing off (hard) and changing direction.
5) When starting out, have them do TEN (round-trip) lengths of the sprint, which should take about one minute. Follow that with one minute off, repeating as many times as they can comfortably. The number of times they're able to do these sprints should be recorded, which will indicate their level of fitness. As they get stronger and fitter, they'll be able to increase the numbers. 

6) As they develop a better level of fitness, have them decrease the time it takes to do the ten sprints, as well as the recovery time, i.e. 45 seconds on, 45 seconds off. 

7) Be sure to have them change their direction with as much power and quickness (push-off) as they can. This is what will make the real difference when (eventually) playing squash-games and matches on-court successfully.

P.S. Explosive Speed Exercises video: https://www.youtube.com/watch?v=FYJJbwG_i8U.