Complex sentence: Marge was so frustrated about the kids that she starts having nightmares, and they decide to take them to a psychologist, who tells them that one of their kids is smart and good (Lisa) and the other is dim and evil (Bart).
Split into simpler sentences: Marge was so frustrated about the kids that she starts having nightmares, and they decide to take them to a psychologist. The doctor tells them that one of their kids is smart and good (Lisa) and the other is dim and evil (Bart).