The child would know the difference the moment they figured out that males and females make babies together. Or when a child watched how male and female animals [all but homo sapiens] abided by that rule overwhelmingly. At that point, a child's mind would begin a downward spiral when told to shut up when asking why just humans upheld homosexuality as a norm when all other mammals do not?
The child would know there's a difference, but not on anything above an observational level. The damage comes, not from a knowledge that two parents are of the same sex, but the parenting styles of same sex households. I have known dozens of children raised by two parents of the same sex. I have been in a position of being able to follow these children to adulthood. The dysfunction doesn't come from knowing that the child has two parents of the same sex and other families don't. The dysfunction comes from being raised in a household where the exposure is only from a single sex. Children raised by same sex parents have the same kinds of emotional disabilities as any child raised in a single sex household, including by single parents.
Historically children have been raised by two caregivers of the same sex. It might be two aunts, or two uncles, or a grandmother and a mother. They don't think a thing about having two caregivers of the same sex. It is only in their relationships with others that the emotional disabilities make themselves known.
Children learn how to relate to the opposite sex by the way their parents interact. A boy learn how to treat women by the way his father treats his mother. A girl learns what to expect from men from her mother. A boy raised by two men will never learn how to appropriately interact with women, UNLESS that gay couple takes steps to ensure that there is a stable woman in that boy's life. A grandmother, aunt, best friend and increasingly, the biological mother. A boy raised by lesbian parents isn't any better off because he just won't see, in a family relationship, how men and women communicate. Children raised in a single sex household either cannot form a stable relationship in a heterosexual context or can only do so with therapy and counseling.
Why don't we notice this? Why isn't this recognized as some sort of national problem? Simple. We live in a pathological society where being pathological is "normal". We have had a couple of generations of divorce and children raised in single sex households. It's not unusual for someone to have relationship difficulties. The exception is a normal human being without need of psychotropic medication.