I would show them a digital clock and a calendar. The clock would cycle through and they would see the pattern of "the fast one goes up to 59 and resets, and every time it resets the next one changes, and that one goes to 59 and resets and the one after that changes, and that one goes up to 12 instead of 59, but oh look, that one cycles exactly twice in one day on the planet. With the calendar, they could see how we keep track of the days, and how we also use the Moon (more or less) to tell longer periods of time.