So ...

I need to create objects and position them in direct relation to a certain object already in the scene. In fact, this item is a parent of said new item-to-be-created.

I have this:

A dialog window.
a scene object.
a view view object.
a parent object.
many child objects.

I want each child I create to use its parameters (ones that I pass to it when instantiated) and create itself taking as 0,0 the parent object's 0,0, not the scene's 0,0.

Example;
I have a parent Square drawn at (4,4, 1000, 1000). Now I want instantiate new items, childs, and if I pass a (2, 2, 10, 10) as parametere, to have it be drawn in what would be (4 + 2, 4 + 2, 10, 10) from a scenes perspective or just (2, 2, 10, 10) from the parent's perspective.

I'm trying to do something like

parent = new QGraphcisItem();
child = new QGraphicsItem();

child->setParentItem(parent);

the thing is, now I don't know how to proceed after this, or if this is even alright.

The reason why I want to attach childs to a parent is because I can just make the parent's size escale with the scene's scaling-resizing and have the children react with their parent and it's own coordinates and proportions.

Think of a graph in a window. If your window changes sizes, the graph rescales everything in accordance with the window's new parameters of height and width and, with that, so get the children to change.