Measure the Moon
Has your kid noticed how much bigger the moon looks when it's low in the sky? What's going on? The moon isn't changing size or getting closer; it's actually just an optical illusion. The moon takes up just about the same proportion of our field of vision no matter how high or low it is relative to the horizon.
If your kid doesn't believe you, here's a simple activity you can do to show him that the moon is always the same size. All you need is a paper clip to get started.