Go in with your eyes wide open. Look at a calendar with your child and count the number of work times that she has to complete the report. Together, assign all the tasks to certain time periods, keeping due dates in mind. This exercise in time management will help your child realize the size and scope of the report project and teach her a successful way to approach long-term projects. Pare a topic down and keep it simple.
Teachers assign reports in different ways.
If you're lucky, your child will have an outline to follow along with frequent check-ins with the teacher. If not, you may need to help pare the topic down to something your child understands. Last year when one of my fifth graders was researching electricity, it became obvious that a large amount of information dealt with concepts too advanced for a ten year old. Frame the report as a series of questions. Too often kids just read and take notes.