There's two camps I've seemed to encounter in youth training when it comes to strength training in young athletes. One camp claims young athletes need to master every movement pattern before they add weight to it. The other views weight as work, and "kids today need to learn to work hard." Both of these lines of thought, in my opinion, miss the mark. To say we should we rush the development of young athletes and add load in the name of hard work is irresponsible, and to say we should only load young athletes once they have mastered a movement is to say the human body is fragile.
I want to break down the second camp first, as it is common among non-strength coaches who run weight rooms. Largely, and this is a personal bias, I see this among coaches in male dominated, contact sports like football and wrestling. They see the weight room as hard work, and believe force is king. They also believe time under the bar will cure any athletic deficits they see on the field. The problem with this line of thought, is it pre-supposes kids only have to work hard and lift more to improve, when realistically, development takes a lot more than this. Kids need exposure to weights, but they need to have a graded exposure in order to have time to build skill and create meaningful adaptations in the weight room. If they are not given this time, and are exposed to more stimulus than they are ready for, the process of adaptation is blunted over time. Essentially, too much stimulus applied too soon leads us to a point where the demand for adaptation limits the height of the peak they can achieve.
But then there's the flipside. What do we do with the people who believe kids need to master bodyweight movements before adding ANY load? Are kids really so fragile? To believe so would be to forget they'll experience double bodyweight forces jumping off the couch onto the living room floor. To then couple it with experience, what happens when the tall lanky kid who can't Air Squat to depth can go A-to-G when you put a bar on their back? Do we take the bar away? Part of the problem with this argument, and what I'd like to point out, is how being too slow to add load may limit the work we can safely get done while creating skill in the weight room. Especially for growing kids, added load often serves as some form of counterbalance or stimulus which actually aids getting into better positions. If we place it well, the external load can be the thing which enhances their ability to make progress. Another great example may be in this, but what is a Bench Press? It's a Pushup on your back. The mechanics are very similar. So what do we do with the overweight young athlete who can't do a pushup but can Bench the bar easily? Do we not let him master the movement until he can do pushups with his body weight?
The problem with putting yourself in either of these camps, per se, is how neither regard the individuality of training or how much nuance there can be to training kids. Some kids need load to move better, and some need to work harder. Some kids need less load to move better and to create time to build a broader base of adaptation. Not all young athletes are one and the same. The key is identifying what they need specifically.
Now, in practical application, we need to be willing to fail a little bit. If a kid is struggling to get into a position, add load to it to see if they can get into it organically. If not, revert it back or try a different loading mechanism. On the other spectrum, if you see a kid whose losing movement quality as loads increase, drop the load down (this should be simple enough to explain). You may need to add up to a point in which the breakdown happens to find what the tolerable limit it, but chances are one bad rep is not going to be the end for a young athlete.
Long story short, don't fall into a camp. Do what's right for the athlete. Don't blindly adhere to a single mantra or dogma about training. Find what works, experiment (and fail) a little bit, and keep making sustained progress towards the end goal.