Collection Tree Display all collection childen items

Hello, lets pretend we have collection tree:

  1. Parent Collection 1
    a. child collection 1-1
    b. child collection 1-2
  2. Parent Collection 2
    a. child collection 2-1
    b. child collection 2-2

I would like to display only the items from collections which parent is ‘Parent Collection 1’

Could you help me with this?