Here's a fun two-minute video that explains the moon's power over the tides.
This one may be helpful for older students, but combined with the English accent, it may be too hard to understand--so I recommend following it with this soundless 20-second blip from NASA. What you might do instead is use your own cookie and rubber band and orange at home to demonstrate! Much more fun. If you need help, this video is a clear explanation, but quite frankly pretty boring for kids.
Here's related project that you might elect to complete in your home this week, though it involves drilling two holes in a plastic bowl.