Chield Serial

Hi.
Is there a way to make a serial columm that works with father table???
I.e:
I have created a table called "vending" and its chield table called "items".
The items table have a serial columm, but I need the serial colum starts with 0
for each vending.

create table vending (
id serial primary key, --Ok, this serial: 0 - 9999xxx
date_ date
);
create table items (
vending_id references vending,
items_id serial, -- THIS SERIAL NEED to start 0 for each vending_id
primary key (vending_id,items_id)
);
insert into vending values (1,now());
insert into vending values (2,now());
insert into items values (1);
insert into items values (1);
insert into items values (2);
insert into items values (2);
select * from items;

vending_id | items_id
1 | 1
1 | 2
2 | 3 <<=== Here! The items_id need to be 1 (start again
for each vending_id)
2 | 4 <<==
Thanks all.