Responsive Menu > Sub item is not showing

The comments at the top of your Child Theme style.css file are not right:
https://projecta.sggraphics.nl/wp-content/themes/quickstrap-child/style.css

You’re missing a “Template” – which should be the name of the parent theme folder: https://codex.www.ads-software.com/Child_Themes#How_to_Create_a_Child_Theme

If you continue to have issues, can you activate your Child Theme?

Thanks, for so far, the Childtheme is created and activated.
What’s the next step to see the submenu-items?

Copy the ‘nav.php’ file from the parent theme and put it inside your Child Theme folder.

Then edit the ‘nav.php’ file, from within your Child Theme, and replace all of its contents with the code from this Pastebin page: https://pastebin.com/Pwz7mHch

Yes, I’ve done that. And for so far the subs are visible. Thanks.
Can you explain what part of the php make this difference.
And, to make them only visible after clicking on the parent is just modify the CSS?

The WordPress function that generates the menu will not have a limit on how many levels the menu goes, but it does have an option if you want that limit. The theme used the option. The option was seen in the code as this:

'depth' => 1,

And, to make them only visible after clicking on the parent is just modify the CSS?

No, that requires JavaScript. A normal menu would involve hovering and not clicking. Hovering requires CSS.
E.g.:

.nav ul ul {
    display: none;
}

.nav li:hover ul {
    display: block;
}

Thanks, By comparing I saw the difference ‘depth’ => 1,
But hover behavior doesn’t exist on touch screen devices?

Touch screen devices may intuitively have a way of dealing with this – I’ve noticed that you have to double-press/ tap.

Oke,
The given css does not hide my subs. I put this in de style.css of the child template.

This works:
.sub-menu { display: none; }
.current_page_item .sub-menu, .current_page_parent .sub-menu { display: block; }
I think this is the end of the topic because there is a solution now. Thanks for helping.