Twenty Twelve based Child Theme Sub-Menu – not working

We just gained a new customer, who’s site was already built in WP. The original designer created (what seems to be) a child theme, based off of the WP Twenty Twelve theme. They seem to have (essentially) copied all of Twenty Twelve files and renamed it. But, they also changed the entire Style.css file – screwing up many settings/styles for the design.

The issue we’re running into now – We can’t create Sub-Menus, properly.

Here’s the site: https://ohioprobateestate.com/
The Fudiciary Litigation page is supposed to be a sub-menu to the Probate Litigation page, as seen in the navigation menu.

But, every time we add the page as a sub-menu in the “menus” section of the admin, the sub-menu is visible, in a secondary row, rather than hidden until rolled over.

I’ve tried adding the navigation css styles back in (below). But this didn’t resolve the issue:

/* Navigation
-------------------------------------------------------------- */
.site-content nav {
	clear: both;
	line-height: 2;
	overflow: hidden;
}
#nav-above {
	padding: 24px 0;
	padding: 1.714285714rem 0;
}
#nav-above {
	display: none;
}
.paged #nav-above {
	display: block;
}
.nav-previous,
.previous-image {
	float: left;
	width: 50%;
}
.nav-next,
.next-image {
	float: right;
	text-align: right;
	width: 50%;
}
.nav-single + .comments-area,
#comment-nav-above {
	margin: 48px 0;
	margin: 3.428571429rem 0;
}
.main-navigation {
	margin-top: 24px;
	margin-top: 1.714285714rem;
	text-align: center;
}
.main-navigation li {
	margin-top: 24px;
	margin-top: 1.714285714rem;
	font-size: 12px;
	font-size: 0.857142857rem;
	line-height: 1.42857143;
}
.main-navigation a {
	color: #5e5e5e;
}
.main-navigation a:hover {
	color: #21759b;
}
.main-navigation ul.nav-menu,
.main-navigation div.nav-menu > ul {
	display: none;
}
.main-navigation ul.nav-menu.toggled-on,
.menu-toggle {
	display: inline-block;
}

Anyone have any suggestions?

I’ve tried deactivating plugins, no success. (But, the original designer also added plugin css styles to the “styles.css” – maybe an issue?)

I’m lost. Please help.