If a kid (or adult for that matter) is learning a new move, how many reps would you say it takes before the move becomes "2nd nature"? It seems to me it is in the hundreds. Obviously it will vary by kid. But something like 300-500 reps should do it.
I truly don't know, but through the course of many camps and skills sessions I've seen many kids from age 5/6 until they are in their teens. If it were a matter of hundreds of reps, which this population got within one year, then they'd do 'em all in games. But it's not what I've seen play out on the field. Players gravitate to 1-3 of their favorite moves 95% of the time. It's rare that I see a player I've known for years do something that makes me go, "I've never seen them do that move before". It's surprising to the defender, because they probably haven't seen it but players are creatures of habit. In the Coerver system there are stages: no pressure, shadow pressure, full pressure. I think shadow pressure stage is the most important, because it imprints the movement in the brain against realistic visual cues. You have to train your shadow defenders to play realistically sans the tackle. This doesn't have to go on forever, may 2-3 minutes in a 1v1 setting then go to full pressure. If they try it and it works. They are likely to come back to it. We like to give audible feedback (like a "Wooo!" if he smokes a defender). I think it's an added bit of positive reinforcement. The value of lots of unopposed reps maybe in that it teaches the individuals how to move like soccer players (coordination, balance, agility). Helping them become more dynamic movers, which is at the core of good 1v1 play. This is all anecdotal. Take it for what it is.
If I am guessing correctly, is shadow pressure essentially where the defender is "going lightly"? They are applying say 50% intensity/pressure? What you say is sage advice, if I'm understanding correctly, about imprinting movement against visual cues. This is something where even a parent who hasn't played much can assist their kid, yeah? They can provide the "visual cue"?
I think shadow pressure is where the defenders step close to the attacker but doesn't tackle. Generally I had players step to about a yard away, practicing their first defender approach. I used "half pressure" which was essentially closing down the attack, making adjustments at half speed. I also used "shadow play" which had no pressure, just a defensive pressence. Really the general principle is pretty simple. Make the training challenging enough to induce positive adaption, but not so challenging as to make success impossible. Adjusting the defensive pressure is only one way of adjusting exercises. I rarely used shadow play. When I did use it, it was like a chalk talk, and I used it to introduce new tactical topics. For instance when I first introduced the 3-line 11v11 system building on the previous SSG exercises working from 1v1 to 1 line and then 2 line exercises.
Yes, I often times step in as the defender, because I know what each kid needs in terms of resistance. Or if I'm trying to get them to pick up on a specific cue. For example, if a defender is on your right shoulder and you want to do a pull back—most times you want to execute it with your left foot (furthest from defender). Unless the defender has overcommitted then the near foot is okay.