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:

i have:

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:

i want have 1600

update:

if I do this it returns me all those associated with all the posts instead of having a foreach loop I would like it to return me specific for all the posts

if I do so it returns me all those associated with all the posts instead of having a foreach loop I would like it to return to me specific to each post.

Show me the third level of taxonomy associated with each single post for the searched taxonomy “sito-web”