An excellent video for all ages: TED Ed’s Reasons for the Seasons.
Why do some regions experience full-time heat while others are reckoning with frigid temperatures and snow? And why are the seasons reversed in the two hemispheres? Rebecca Kaplan explains how the shape of the Earth’s orbit around the Sun and the Earth’s tilt on its axis affects the amount of sunlight each region receives.
Related reading: Kepler’s laws of planetary motion.
Plus, check out Planetary Orbits: Not a Perfect Circle, which includes some drawing ellipses activities, and these related Earth and Space Science – Climate and Seasons lessons for grades 7-9 from NASA.
There are more videos about or including the seasons on this site.