When running fast, some athletes, without having been taught, will demonstrate good sprint technique. A coach should do his/her due diligence to develop an overall decent understanding of the Key Performance Indicators (KPIs) of sprinting, which hopefully includes knowing what KPIs are of primary importance, and which are of lesser importance when observing young athletes. At the top of the list of KPIs is having good front side mechanics. Some athletes, without being told anything, are better than others at keeping the body segments in front of the body while sprinting effectively, while minimizing any action that occurs to the back of the body, ie; when running, the thigh of the leg that is pushing off will travel no more than just a little bit past being in line with the torso. This is a visual sign associated with having good front side mechanics. It is important to realize that when the body is in a leaning forward position during acceleration, that the torso is still a reference point, and the thigh of the leg pushing-off still shouldn't travel much past being in line with the tilted forward torso.
For athletes that don't have good front side mechanics, the question should be why? Since good front side mechanics should occur from the start of the run, the coach needs to examine the start and initial strides. Often times the first and/or second stride will land too far to the front and will require over-pushing to the rear, thus beginning a pattern of excessive actions to the rear. In addition, good arm action can lead the entire body in its effort to shift the body movements to the front of the body. Consult my Starting and Accelerating blog for detailed specifics.
A lot of speed fundamentals may be addressed within the warm-up period. My Speed Enhancing Warm-up Procedure blog covers this, with primary objectives being to help develop; effective muscle recruitment patterns and efficiency of movement when sprinting, good frontside sprint mechanics from the start, strength/power and ranges of motion in key areas, and to learn basic race models for sprint distances. Falling short in key areas such as these don't allow the athlete to continue to improve and blossom as he/she is capable of. A premature leveling off of performance is likely to occur instead.
When coaching/training young or fairly young athletes, in regards to physical abilities, it is good to cover all the bases in an age appropriate way, ie; speed, strength, agility, and age appropriate plyometrics, ie; skipping for height and for distance. No need to go crazy with it, just work it in on a fairly regular basis. Mix these abilities with appropriate attention to the fundamentals of the sport, and you can have some very good results as the athlete grows and matures.
The trainer for Overtime Athletes has a good video for hitting the bases in regards to a speed program. I like everything he says except the part about not having rotational movement during sprinting. Obviously there are linear elements to sprinting, as the athletes are trying to get from point A to point B as fast as possible, and the shortest distance between two points is a straight line, however; Dan Pfaff stated "It is important that all coaches understand that sprinting is a rotational-torsional activity", and he describes the slight oscillation and undulation of the hips and shoulders in this video. Efficient sprinting is hip based with a quiet spine, you do, however; need to be able to distinguish between what is acceptable and what is not acceptable. In these 100 meter dash races you'll see a variety of examples within the bandwidth of acceptability.
As for general strength, initially I believe that a variety of bodyweight exercises can suffice for youths. In time, learning to pick things up from the ground properly, as well as some basic exercises with weights, medicine balls, kettle bells, etc... is a good thing, and by high school, some more advanced lifts like "cleans" are appropriate. Allowing youths to play a variety of sports, at least early on, is also very important for overall athletic development. When to have youths focus on one or two primary sports is a debatable topic.
I don't believe it is necessary to get extremely anal with youths regarding technique. More than anything the coach needs to understand what is major, what is not, and of course focus on the things that are major. Get the athletes on the right path and in the ball park with solid fundamentals and good training without going overboard, and with time, special performances will come.