IIRC a circular graph would typically be done via pool-specific access types.

    -- Forward declaration.
    Type Element(<>);
    -- Assuming there's a Graph.Pool implementation of the base Storage_Pool object.
    Type Pointer is access Element
       with Storage_Pool => Graph.Pool;
    Subtype Handle is not null Pointer;
    Type Children is array(Positive range <>) of Handle;
    Type Element(Parent : Handle; Child_Count: Natural) is record
       Data : Integer; -- Or whatever your actual data would be.
       Link : Children( 1..Child_Count );
    end record;