nextval on insert by arbitrary sequence

I'm not sure if I worded the subject right, but my problem is this:

I have a few entries in one table. Each row is the parent of many
entries in a second table.
In the second table I have a lot of entries referencing the entries on
the first table.
So far so good. Basic foreign key thing.
The entries on the second table need to be numbered, but instead of a
single sequence for all rows,
I need a sequence per group of rows, according to their parent record.

I have some ideas but they are not coming together.
I thought of creating individual sequences for each new parent row, and
maybe store its name in the parent row itself, so it can be accessed by
it's children row.
But then, how do I get a field on the second table to DEFAULT to
nextval() on it's parent's sequence?

INSERT using SELECT?
TRIGGER?
RULE?
FUNCTION?
Is it best to handle this things entirely on the client ?
What is the approach for this problem?

Thanks.

PS: Hmmm... It doesn't look like I got my terminology right. Sorry for
that.

--
Dado Feigenblatt Wild Brain, Inc.
Technical Director (415) 553-8000 x???
dado(at)wildbrain(dot)com San Francisco, CA.