It isn't.

It's funny because you don't expect kids to want there to be monsters under their beds. Typically, or rather trope-ically, children ask their parents to make sure there are no monsters under their beds and at first it would appear the same is true here. However, the child and parent themselves appear ghoulish, and when it's revealed that there is in fact a monster under the bed, ostensibly the father as it is similarly ghoulish and tells a pretty decent double-layer dad joke, the child is relieved. It's irony! And puns. And just good Halloween vibes, which I admit aren't inherently humorous. But I'd argue they are almost always inherently light-hearted. I give it 9/10, especially compared to a lot of the shit I see posted around here.

Same reason most things are funny, or at least mildly amusing: inversion of expectations.

It's called a flashlight, or a "torch" in more primitive parts of the world...jk UK, love you crazy kooks.