Parents should help their children complete homework by providing a clean and quiet environment conducive to work. Helping their children brainstorm for projects is also acceptable. However, answering questions for them is more than what a parent should do.
One of the reasons why teachers assign homework is to help keep parents involved in what their children are learning. It's important for parents to understand that their children have homework and what the teacher expects on a given assignment. However, understanding that children have homework and sitting down and completing the assignment with them are two different things, and doing their work with and for them is not helpful.
Another aim of homework is to give students the skill of completing work independently. It's one thing to work on an assignment in class, as part of a group, or with the teacher right there to show every step of the way. Homework instills independent learning. The lengthy process of putting together a science fair project, going from initial conception to design and completion, teaches students how to complete a long-term process involving multiple steps. When a parent takes over the project and completes it, the student might win a prize, but he has not learned how to complete that sort of undertaking.