I am a certified teacher, and I have been teaching for the last four years. I think the biggest complaint that I have ever received was that there was not enough time in the day to do homework with my kids.
I teach job skills at a high school level right now, so I do not assign homework. I have taught students from the fourth grade to the twelfth grade, and I know I am a great help.
I charge based on the difficulty of the subject and on the amount of help needed on homework.