One of the bigger misconceptions people have when it comes to Youth Training and Development is how more training equates to better outcomes. The more reps we do, the more sessions we do, and the more weight we lift, the better the performance on the field will be. Realistically, this is far from the truth.
Let's be real, most of us have done some form of training plan which had us doing 3x10 on every lift for a while, then 4x8 on a lift, then 5x5, then 3x3, and then tested a max weight. This is not necessarily wrong, if your goal is to be a bodybuilder, and you're only utilizing machines and isolative exercises. But if we're trying to train movement and skill within the context of the weight room, we need to understand how little youth athletes need in terms of volume at the onset of training, and how the total volume should progress over time.
In simplest terms, the timeline of volume should look like a Bell Curve. New athletes in the weight room should be doing lower amounts of reps per set, then increasing reps per set as they improve in skill, then later return to low reps sets as they master movements and drive higher outputs on exercises. This is for several main reasons:
Young athletes have a low threshold for consistent movement. Like any skill, the first few reps or attempts at it are going to look far from the ideal. But just like a local tells you to wait five minutes for the weather to change, wait one or two reps and you'll see a nearly brand new movement. As athletes develop skill, they change rep to rep, and have a low threshold for consistency in how a movement is performed. For young athletes, say those in Middle School, most often the tolerable limit of reps before major changes in form or technique occurs between reps 6-8. For this reason, performing sets of 4-6 tends to be a more viable option to develop skills and general baseline strengths before a potential injury or poor habit development can occur.
Young athletes need to make movements consistent before they add load. As they learn new skills, young athletes (thinking more Middle School and early high school) will build consistency and skill within an exercise. As skill is developed, the total reps of sets should increase from 4-6 up to 5-10. This will be largely dependent on the skill level, and built up over time. One shouldn't jump from 4-6 reps a set to 10 reps. A gradual progression up to 5-10 reps should be utilized.
Young athletes don't have the technical ability to handle heavy loads or loads under higher fatigue. The reason the shape of the total training volume looks like a Bell Curve, is for later force and power outputs to truly take shape. As athletes develop more skill and consistency under fatigue, then output can be driven by heavier, low-rep sets. This looks more like heavier sets in the 1-5 range. Young athletes often can handle higher loads than what they're movement dictates, but to do so under fatigue would be disastrous.
Now, in a group setting, making generalizations for a specific group or set of athletes may require open ended sets. This may be a coach telling an individual athlete to "add a rep" during a set. It may look like actively progressing volume over several weeks or months while maintaining the exercise selection. In an individual setting, it can look like actively changing and manipulating a program to reflect an athlete's current skill level in a given exercise. I have plenty of athletes who get sets of 6-8 reps on some exercises, while only getting 4-6 on others. This allows them to develop movement skills at their own pace in relation to the skills.
Now, to revisit the initial point about more being better, I'd like to consider the consequences of doing too much at the beginning. Skills are not best built under states of fatigue. Doing so leads to poor execution and poor habit development. This is both an inter-workout issue and intra-workout issue. We don't need "more" training sessions each week. We need quality training sessions each week, where movement skill and strength is built in a consistent manner. The more we develop under fatigue, the greater chances we have of developing poor habits and skills. These poor habits then have to be corrected down the road in order to "course-correct", or lead to a stagnation in progress. Neither of which is desirable.
If we, however, start slow and low with the intent of building skill over time, we not only build adequate skills, we end up providing a better opportunity to excel down the road when loads can get heavier.