Dear young friend,
Have you ever noticed that the moon looks different at different times? Sometimes it's big and bright, and other times it's small and thin. This is because of something called the moon's phases, and it's all because of how the moon moves around the Earth and the sun.
Think of the moon as a big, shiny ball. When the sun is shining on one side of the moon, we can see that lit-up side on Earth, and it looks full and round. But as the moon moves around the Earth, the way the sun's light hits it changes, so we see different parts of it lit up. When only a little bit of the lit-up side is facing us, it looks like a thin crescent. And when none of the lit-up side is facing us, it looks like there's no moon at all.
So, the moon changes size because of how it moves around the Earth and how the sun's light shines on it. It's kind of like when you're playing outside, and depending on where the sun is, your shadow looks longer or shorter. Just like that, the moon's size changes for us because of how it moves and how the sun's light hits it.
I hope this helps you understand why the moon changes size, and I'm always here if you have more questions!