Depending on the topic, it can be hard to come up with an experiment, which asks a question and finds its answer. Sometimes measuring results and correctly attributing them to a factor can be difficult. But, the "exhibit" category of projects could be a solution. For an exhibit, you need to find something interesting about the subject and explain it. If you child loves music, perhaps you could explain how sound is created by a piano. If your child loves jumping rope, perhaps you could explain why holding your arms still while turning the rope with only the wrist causes the rope to rotate faster.
Utilize the websites listed above to adapt ideas to your child's interests. Try Science Buddies which provides a "Topic Selection Wizard" to help you find a science project which suits your interests.