Is this the exploratory type of science project or the "form a hypothesis, conduct an experiment, etc" formal type of project? At that age, they generally do an exploratory type, and that should be based on the child's interests.
At that age, my daughter was very into fossils. She'd pick up brachiopod fossils all over the place. We went to a science museum where she learned our area was once under a shallow sea. She learned how geological/archeological digs are done, and how wetting the rocks make the fossils more visible. She taught ME these things because she was so interested in this.
So, for her science project, she made a little "dig" for the other students to try, and all I had to do was provide her with the materials she wanted - string, paintbrush (for "painting on water."), sand, etc.
It was written by her - misspellings, backward letters and all (back before everyone did everything on computer).
I hate the type of science projects some other schools want the young children to do, which is experimental rather than exploratory.
An easy experimental type one of my kids did was after wondering if there was something about the bread bag kept the bread fresher longer, or if a cereal bag would work better. So she then tried out several different types of bags and found in which the bread turned moldy last.
In either case - it should be something your own child wonders about, and wants to explore.