Generic child units

I need some enlightment in this area... Let us suppose that I have two 
generic packages declared in separate files and one is the parent of the 
other:

generic
    type Elem is private;
package Parent is
    type Vector is array (Integer range <>) of Elem;
end Parent;

generic
package Parent.Child is
    subtype Vector2D is Vector (1 .. 2);
end Parent.Child;

How can I instantiate the child package within the declarative region of 
the parent package (as required by RM 10.1(18))?

Rodrigo