Traditions or not, the role of a child doesn't "by default" include any script for interaction, even as an unwilling observer, with the parents' sex life.

Why should a child have a predefined role or script?

People instinctively appear to see familial and sexual intimacy as two separate kinds of bonds, and the mind-screw that comes with mixing them might be one of the reasons for having incest fantasies. Such a mind-screw could easily be discomforting/unpleasant in everyday contexts!

People also instinctively appear to see men and women as two different kinds of people.