I guess it depends on who is having sex. If that's her mom and dad, she might be upset that daddy is hurting mommy and harbor anger towards him and then all men. If that's her mom and some random guy, she might think he's hurting her, or she might come to equate sex with power and try to use sex to get what she wants from men.

You're a guy right? I imagine it's not as bad for little boys as it is for girls to see sex at a young age. Look at the expression on the little girl's face and tell me she's not disturbed by what she's seeing.