SciShow Kids offers a nice video that can help K-3 students understand why the length of daylight changes throughout the year.
Reasons for the Seasons is a TED-Ed lesson appropriate for upper elementary and middle school students. The lesson explains the relationship between the shape of the Earth's orbit around the Sun, the Earth's tilt on its axis, and how those affect the amount of sunlight on different areas of the Earth.
And for a little perspective on winter vs. summer solstice here's a great side-by-side time-lapse of the winter and summer solstices in Manchester, England.