How to list down tags

In your theme, in functions.php try some stuff in their, or css MAY work.
Create a class and add font-size:xx;

AFAIK, there’s no parent/child in Post Tags. Use include instead?

To list post tags using that function, add taxonomy=post_tag

or wp_tag_cloud() ?

https://codex.www.ads-software.com/Function_Reference/wp_tag_cloud
you can set the parameter to influence the sorting and the sizes.

there is no child-of parameter, thought.