Are you attempting to be able to show and hide the Crosshairs as the player aims in and out of his weapon?
If so you need to rethink your logic. Event BeginPlay is only called once when play begins on the level. So in essence it takes a snapshot of your AimButtonDown boolean variable at that particular frame. It is not capable of detecting when that variable changes.
If you manually change that variables value on your Player, you should see how it affects the drawing of the crosshairs.
You need to change your logic such that, when the player uses an input event (Right mouse button for example) to aim down their sights, this is where it would be more appropriate to decide to show or hide the crosshairs.