All About Bears). When she's a little older we'll worry about covering more of the details. In the meantime, if you have elementary school students who are wondering "why do we have fall?" here are a couple of good little videos on the topic.
Why Are There Seasons? from SciShow Kids is a good video lesson about seasons. The video is appropriate for students in primary grades.
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.