Retrieve the child terms by having the parent’s information

I have access through a series of joins to the terms table, in order to bring out all the taxonomies that are associated with the post if I wanted to.

with this code:

<?php
echo "{$r->term_id}";
echo "{$r->name}";
?>

i have:

22
Sito Web

Sito Web it s a children taxonomy servizi_pro

how to retrive only the children about: Sito Web

i want only the childern from: $r->name

schema:

servizi_pro
|
|_ Sito Web
   |
   |_ 1600

i want have 1600