For us, when we tried to "step playside and block your gap/cylinder" we lost all our aggression. Kids seemed to step and search and never getting any movement on defenders, "catching" them, if you will. Perhaps we weren't teaching it right and that's OK, we just
Which is why we give them a "who to block" on every play. Get mean and nasty and GO GET HIM, aggressively. But we teach them that they are only responsible for "half of the defender" thus, if he slants away from your playside gap, you wont have to worry because you know your teammate is also in GO GET HIM mode, aggressively.
This is where we have to put in the DON'T CHASE rule, because if you are uncovered and the DL you GO GET slants away, we had issues with kids chasing him out of his cylinder, allowing LB run-through.
To be honest, many DL in our youth league align in gaps and/or aren't super active in slanting when aligned head up. So when they are gapped or heavy shaded, we get great double teams and we work those to get movement. IF we get good at the double, we then add in "if you have a LB within arms reach, come off and block him". but that is a big IF.
If we just get really aggressive on the double teams, and teach the back to read the LBs it is his responsibility to make them wrong. take the other lane and GET US 4.