Getting de ID form a Category child

Hi guys, i have a problem and i think that most of us have the solution..

I want to get all of the category id’s from a category parent..

for example:
ID NAME
—————-
26 Musica

8 — Conferencias
310 — Cursos
243 — Exposiciones

139 Tipo

4 — Conferencias
10 — Cursos
23 — Exposiciones

The main idea is a query to get all of the category’s ID included en the category ‘Tipo’ or ‘Musica’

something like this

function GetIdChild(139)

and returns

$list=’4,10,23′;

i have been tried with

get_the_category();
wp_list_categories();

but i don’t have results… ??