Check if your cross hair is actually visible (maybe you are looking at it from the side or at the backside of it or...). Then check it's location in object space to see if that matched object space of the weapon (more or less). Making something a child of an object does in no way meant that it appears in the same place. If the parent is located around 0,0,0 in object space but the child is located around 100,1000,10000 in objects space, they never appear on screen together even if the child inherits the transformations of the parent.