For warm-up runs, ideally; you want enough room for the athletes to be able to achieve a top speed that is appropriate for him/her for the sport, then be able to decelerate under control (injury prevention). For track and field and sports that are played on a big field, ie; football and soccer, when the athletes are in good enough shape to sprint fast, 50-70 meters is likely far enough. When athletes are at early stages of conditioning, encouraging athletes to extend their acceleration the maximum distance can be overly stressful to the legs, and muscle strains and pulls can result, especially if done repeatedly. For these athletes, I like to encourage them to get out with a decent start, continue with a decent acceleration, and allow the speed to level off at a point in the run that they feel it is best to do so, then allow themselves to gradually decelerate, not being overly concerned with competing.
I also allow the athletes to begin their warm up runs without having to be perfectly still at the start to save the legs from some wear and tear. I like to give rhythmic commands without a lot of time between each command, ie; "to the line", "runner's set", "go", to allow the athletes to anticipate the "go" command. I'm fine with the athletes moving as I'm saying "go", or even starting a little before "go". Again, the aim is to ease the stress on the legs. For track and field, sometimes warm up runs can be 100-150 meters to practice to prepare for certain tempo workouts.
At some point however; the athletes do need to be able to pause, be basically motionless for at least a moment in their starting position, and then explode into the run. This allows the athlete more insight into learning how to utilize leverage and to create momentum while in accordance with the rules of racing. Michael Johnson explains in this video how a motionless pause allows for better power if the body is positioned properly. Always rolling into the start can also take from the athlete's ability to generate tension where necessary, ie; the ankles. (See Starting and Accelerating blog) Although in many sports athletes are asked to accelerate while they are already moving, learning the basics of accelerating from a stationary position has very valuable carry over.
Observing how the athletes respond to the starting commands when in a standing start position, and how they begin their runs from a standing start is a big deal. There should be adequate bending at the hips and knees to promote good glute recruitment, as well as an adequate forwardness in relation to the starting line. Females more than males have difficulty bending adequately as is explained in the first paragraph in this blog. The coach should also understand the subtle difference in footwork that he/she is likely to see the athlete perform when allowed with sort of a roll into a standing start, as opposed to being required to be stationary prior to the "go" command. (See my Starting and Acceleration blog.) Starting with sufficient bend and good footwork will allow the athlete to maximize the push into the first step and set him/herself up for the all important second step which should lead to a good third step and rest of the race. All of which should feature good front side mechanics. Once again, check out my Starting and Accelerating blog!
Typically I'll tell the athletes for the first warm up run not to run too fast because they are just getting loose, maybe 50-60% intensity or so, and with ensuing runs increase the effort. For younger ages, however; if you start them at the same time as other teammates, they will simply race each other, running as fast as they can. To minimize the urge to compete, you may choose to stagger the commands so the kids may start running at different times, until you do want them to race each other.
It is very important for the coach to be knowledgeable and be able to say the right thing at the right time. I'd rather say too little than too much. It is a lot easier to help an athlete to continue making progress in baby steps than to get him/her back on track after having been bombarded with cues and led astray.