I am trying to find a manga (I'm fairly sure it an isekai) where the main character wants to test out his magic so he tries to shoot down a dragon that is far away. He shoots off his magic and ends up destroying one of the planets blue moon.
Addendum: there is a second moon that is, I think red, that comes out unscathed. Also this manga might be a harem type. the MC is not a child, though he may be reincarnated and grows up, I can't remember. He meets a female knight that comes to investigate if demons or the demon lord had something to do with the destroyed moon. I don't think he is a demon lord but maybe I don't remember.