What are the contents of your XML?
You need to define the parent template in order the cascading will work as expected. Eg

<?xml version="1.0" encoding="utf-8"?>
<extension type="template" client="site">
  <name>somename</name>
  <version>1.0</version>
  <creationDate>July 2020</creationDate>
  <author>dGrammatiko</author>
  <copyright>Copyleft</copyright>
  <description>Demo template to test Child templates PR</description>

// This part is ESSENTIAL
  <inheritable>0</inheritable>
  <parent>theNameOfTheParentTemplate</parent>
</extension>

you can check this child template https://github.com/joomla/joomla-cms/files/4977974/sitechild_v2.zip

that is inheriting from https://github.com/joomla/joomla-cms/files/4977972/siteparent_v2.zip

Hi,
My main it's to overide some part of the template ATUM.
My XML is

<extension type="template" client="administrator">
	<name>xxx</name>
	<version>1.0</version>
	<creationDate>2021</creationDate>
	<author>NAP</author>
	<description>Template ADMIN xxx</description>
	<parent>atum</parent>
	<inheritable>0</inheritable>
	<media destination="templates/administrator/xxx" folder="media">
		<folder>css</folder>
		<folder>scss</folder>
	</media>
</extension>

@NetAssoPro sorry but the child templates EXPECTS the static assets stored at the media folder in a path like: media/templates/admin/templatename or media/templates/site/templatename check the provided templates above.

It's sad :(
You lost all the advandage of overiding if you need to copy main files in a directory to override them in an other diretory...
You don't think so ?

You lost all the advandage of overiding if you need to copy main files in a directory to override them in an other diretory...
You don't think so ?

Not really. The child templates should work only with compatible templates. Atum is not inheritable (yet), check the xml file, there's no line <inheritable>1</inheritable>. This means that you shouldn't create a child from this template. So basically you tried to do something that we explicitly wanted to avoid: conflicting old templates with the new ones (that support Childs). This would been easier if the GUI part was in place (I'm still working on it, although it takes longer than I expected)

Ok, i understand. Really thanks for your fast feedback. I will let my hack ,waiting the ATUM inheritable.
I'm not sure the GUI will be enought in my case, i need some advanced things (like use Fontawesome pro instead of Fontawesome free) and extend the possibilities to custom template colors.