I now know how to parent Opacity keyframes. I create an expression on the child layer's Opacity, and pick-whip to the Opacity of the parent layer. (I only learned this after someone in this forum explained it to me in detail.)
...is it possible to create an expression where, if one layer's Opacity is 100%, then a child layer would become 0%? And vice versa, when the parent layer is 0%, then the child layer is 100%?
When layers 3, 5, 7, and 12 in the comp are on, then 4, 8, and 13 need to be off. And vice versa. (And the layers have varying blending modes, in case it matters.)
Sure, simply reverse-remap the values using a linear() expression.