Hear your child's side of the story. Take everything they say with a grain of salt because remember: they are trying to avoid disappointing or upsetting you.
Evaluate your child's information. If the complaint is about a particular grade, have your child explain the instructions, what they did not understand about the assignment and why they think they did poorly on it. The student could have just simply not prepared enough for the task.
Decide if the complaint is worthy of approaching the teacher about. If the grade is minor and will not negatively impact the child's grade significantly, it is better to not approach the teacher or administrators and come from a place of compassion that acknowledges how much a teacher has on their plate already. If the grade is significant and does in fact seem questionable or your child's complaint relates to something the teacher said in class or to the student particularly, send the teacher a kindly worded email that inquires about the situation and asks for the teacher's point of view.
Approach administrators for a meeting with the teacher and yourself only if the teacher does not respond to your inquiry or answers in a rude manner. If the teacher does provide insight into the situation and gives guidance for how your child can better succeed next time, let the issue go and encourage your child to work a little harder.
Attend the meeting if one is necessary and try to come with an open mind and something kind to say to the teacher. They hear complaints for much of the day most days and get very little appreciation in return. This approach of trying to come to an understanding versus blaming attitude will go a long way in helping the teacher open up about the situation and how your child can better succeed in his or her classroom.