Let me expand a little now that I’m off my phone. Marking the menu up with the sub menu set with its Parent to be the identifier of the Parent menu, is correct. Next it comes down to your implementation of how to make that work. How are you weaving together your css, js, and Hugo template code?
The code in the links below, with:
{{ $currentPage := . }}
{{ range .Site.Menus.main }}
{{ if .HasChildren }}
<li class="{{ if $currentPage.HasMenuCurrent "main" . }}active{{ end }}">
<a href="#">
{{ .Pre }}
<span>{{ .Name }}</span>
</a>
<ul class="sub-menu">
{{ range .Children }}
...
… works, but you need to adapt it to the choices you make in terms of css/js framework.
I think your original attempt looked correct, except for hasChildren = true which is meaningless because this value will be calculated depending on whether the entry has children or not.
However, your theme does not seem to support sub-entries (second-level menus). So I guess your original attempt resulted in the parent entry being displayed properly, but without children. Is that correct?
If so, please open an issue on the theme’s bug tracker, this has nothing to do with hugo or your config
It does not look like the demo site of that theme has any “sub menus”. You’ll need to add them (overriding the theme), or get the theme author to add them.