Group bys and sub-queries

Hi

I have the following two table structures:

create table parent (
  parent_id number not null,
  class varchar2(1)
);

create table child (
  child_id number not null,
  parent_id number not null,
  date_done
);

I want to retrieve the number of child records for each of class A and
class B of the parent records, grouped by date_done.  E.G. -

week_commencing	A	B
30/04/2007	1
14/05/2007	3	1
21/05/2007	9


I am attempting to perform the following query:

select round(overall_child.date_done,'IW') week_commencing,
       (select count(subset1_child.parent_id)
        from   parent subset1_parent,
               child subset1_child
        where  overall_child.child_id = subset1_child.child_id
        and    subset1_parent.parent_id = subset1_child.parent_id
        and    subset1_parent.class = 'A'
       ) A,
       (select count(subset2_child.parent_id)
        from   parent subset2_parent,
               child subset2_child
        where  overall_child.child_id = subset2_child.child_id
        and    subset2_parent.parent_id = subset2_child.parent_id
        and    subset2_parent.class = 'B'
       ) B
from   child overall_child,
       parent overall_parent
where  overall_parent.parent_id = overall_child.parent_id
group by round(overall_child.date_done,'IW')
order by week_commencing;

Attempting to run this results in the error:

   5         where  overall_child.child_id = subset1_child.child_id
                    *
ORA-00979: not a GROUP BY expression

Adding overall_child.child_id to the "group by" clause allows the
query to run, but gives one row per record where I need one row
covering each week.

Does anyone have any suggestions as to how I can achieve this aim?

Many thanks

David