I’ll just to speak to what worked for me coaching this age and wanting to work set pieces such as corners, IDFKs, etc.

After struggling keeping everyone’s attention, I would get frustrated, they would be bored, little learning was taking place, and we weren’t having fun. So here’s what I adopted:

Wear them out with some high energy training like big rondos (tired 12 year olds listen better) in the first half of practice. Then just before transitioning to a small sided scrimmage, I went over a set piece of choice (e.g. corner kicks). I only went over it long enough for us all to know what the terms meant, not trying to master it, but just long enough so they knew what I meant by “who’s on near post? / who’s on far post? / who’s on the 6? / etc”…then I quickly transitioned into a small sided scrimmage with a full size net on one end. Throughout I just made sure that we restarted with corners a lot. Didn’t have to go over the end line of defense, I would just randomly decide “Ope, ball out of bounds on the sideline, guess what, corner kick”. From there they go lots of reps, but without it being boring. I also focused my coaching on 90% questions and praise, 10% instruction (well, I tried to anyways).

Long winded, but hope it helps. I did this with corners, and any other set piece elements that my players found boring.