Seasons happen because Earth is tilted on its axis. As Earth orbits the sun over the course of a year, the tilt causes different parts of Earth to receive more or less direct sunlight. When your part of Earth is tilted toward the sun, sunlight hits more directly and you experience summer. When your part is tilted away, sunlight hits at a lower angle and you experience winter. The distance from Earth to the sun is NOT the cause of seasons.
Use a globe and a lamp (representing the sun) to demonstrate how Earth's tilt changes which hemisphere gets more direct light as Earth orbits. Shine a flashlight straight down on a surface versus at an angle to show how angled light spreads over more area and heats less. Connect to the observation that the sun is higher in the sky in summer and lower in winter.
You know that the year has four seasons and that summer is warm while winter is cold. But why? Many people guess that Earth must be closer to the sun in summer and farther away in winter. That makes sense as a guess, but it is actually wrong. The real reason is much more interesting: Earth is tilted.
Imagine spinning a basketball on your finger, but instead of spinning perfectly upright, it leans about 23.5 degrees to one side. That is how Earth spins -- it is tilted on its axis. As Earth travels around the sun over the course of a year, this tilt means different parts of Earth face the sun more directly at different times. When the Northern Hemisphere (where places like the United States, Europe, and most of Asia are) tilts toward the sun, those places get summer. At the same time, the Southern Hemisphere (where Australia and much of South America are) tilts away from the sun and gets winter.
Why does facing the sun more directly make it warmer? Try this: shine a flashlight straight down at the floor. The light makes a small, bright circle. Now tilt the flashlight at an angle. The same light now spreads over a bigger area, and each spot on the floor gets less light. The same thing happens with sunlight. In summer, the sun is high in the sky and its rays hit the ground almost straight on, concentrating warmth in a small area. In winter, the sun is low in the sky and its rays hit at an angle, spreading the same energy over a larger area. Less energy per area means less warming -- that is why winters are cold.
Here is the proof that distance from the sun is not the answer: Earth is actually slightly farther from the sun during Northern Hemisphere summer. If distance were the cause, the whole planet would have the same season at the same time. Instead, when it is summer in New York, it is winter in Sydney. That only makes sense if the cause is the tilt, which favors one hemisphere while disfavoring the other.