Calling it “sexual morality” is not going to go far to allay the feeling of mistrust that many parents have vis a vis church personal and their children.
My boys are now in their twenties, and there was absolutely no discussion about sex (or “sexual morality”) in their preparation classes. Nor in mine forty-five years ago. I agree that this is best left to the parents, and if they would rather not have their children present when this is discussed by a catechist, I would have to agree that this is their right.
I’m sorry, but the impression I get from the OP is that he wants to be granted a “captive audience” to preach to. Kind of like the evangelists who preach to the line of people waiting at the DMV, knowing that they can’t step way without losing their place in line. He’s also aggrandizing his role, his authority and his mandate, which, as others have said, is to assist the parents. If the parents don’t require or desire his assistance, for whatever reason, he should humbly accept it and butt out.
If the bishop has a problem with this, he can refuse to confirm the children. That his right, and his decision.
Oh, and I read every single post very carefully before I answered.