It's all because of gravity. Whenever we are standing on either the surface of moon or Earth we think that we are standing straight.
So my the observer is standing on the Earth he will think is standing straight on the surface of Earth and will look upward to see moon.
Same goes for the moon. When the observer is on the surface of moon then just like Earth if he look downward then he can only see the surface where he is standing but if he want to see the Earth then he should look upward.

I think it is the explanation you can give your 9th grade student so he can understand it in the most easiest way.