I think parents should help, esp. if they see that the child is struggling in a particular subject. Now this doesn't mean to solve the answers for them, but rather to try explaining the concept better. Sometimes students don't understand the way their teacher is trying to explain the concept, so it's always helpful to get the explanation in a different manner. I know that when I was in school, if there was something that took a while for me to understand, my parents may actually make up additional questions for me to solve, to ensure that I understand the overall concept, not just how to get the correct answer for that one question.
It is always more important that the student understand the concept, not just getting the correct answers. In college when I was taking organic chemistry, there were two professors that taught that course. The one taught things more as what to memorize -- if you have reagent A and catalyst B, you get this type of product C. The other taught more on the underlying concepts -- why does reagent A react in the presence of catalyst B to get product C. He would have us draw diagrams of the chemicals and show how the electrons would move about in order for the reaction to occur. After completing O-Chem, I could tell the difference between the students that had the first prof. vs. the second. Although the students for the first prof. always got high grades in o-chem, their understanding was limited to just what would be on the tests. The students from the second prof. had a better grasp of the underlying concepts, and so were able to use that knowledge to piece together what they learned in o-chem to help with more advanced chemistry classes. In fact since I was a student of the second prof., in grad school I was able to propose a plausible reaction to what we were studying... We knew what the reactant and products looked like, but we were trying to determine how the protein worked. Looking at comparative structure of this protein with others, we could assume that this protein acted in a similar manner. Since the other proteins functioned in a particular way, if I assume the protein we were studying also did the same reaction, I was able to show how the reactant interacting with the protein would result in the product.
Thus I think when parents are helping children, I think it is more important that the parents try to help by explaining the concept in a way that the child understands, and not be focused so much on that child getting the correct answer; the correct answers will eventually come about with a solid understanding of the underlying concepts.