I think you were fine. I might have gone up to the boys at the intermission and talked to them and explained that I was glad they were enjoying the concert, and that they probably didn't realize how loud they were, and how much I appreciated that they were quiet once it was brought to there attention. But I tend to parent every kid in my vicinity (which may or may not be advisable).
This is overstepping. You are not required to explain yourself to people who are not able to follow the rules and basic principles of etiquette, whether they are children or not. At this point it is their caretakers responsibility to explain things further.
If people don't want to be shushed or told to be quiet then they should not behave in ways that create the need for others to do those very things. H will freeze over before I apologize or explain myself to someone for shushing them, when in fact, they owe me
the apology for being noisy. Children don't get a pass from me, if they are old enough to be there they are old enough to follow the rules like everyone else has to. If they are not old enough to be quiet when necessary then they do not belong where quiet is not only a requirement, but a necessity for the performers to concentrate.
If the parents need to point fingers then they should point directly at themselves.