If I understand correctly then your problems are caused by the fact that you are trying to make the missile a child of the cannon (or whatever you have there). The problem is that you are probably handling mouse events in the scene (in scene coordinates) but trying to move the ball in cannon's coordinates. This is of course doable, all you need is to map coordinates from the scene to the item but if you rotate the cannon again (or whatever you have there) the ball will follow the rotation which I don't think is something you want.

To understand Graphics View coordinate system, imagine that the scene is a sheet of paper that has an origin and x and y axis marked on it. Each item is another sheet of paper with its own x and y axis and origin. The origin of the item is pinned to the position of the item on the piece of paper representing its parent. So if you have an item that has its own children, you pin them to the parent item and not to the scene. Now if you start rotating or moving the sheet representing the parent you will notice all children also change their position. It's a good idea to actually take a few pieces of paper (or better yet transparency foils or something that is semi-transparent), a pencil and a couple of pins and repeat all this as an exercise to understand how it all behaves.