I should load the subcategories to the parent category, in this example I suspect a relation called children

$catecories = Category::whereNull('parent_id')->with('children')->get();

And in your blade

@foreach ($categories as $category)
    //html before the subcategories
    @foreach($category->children as $subcategory)
        // the html for the subcategory
    @endforeach
    //html after the subcategories
@endforeach

Edit:
No I read your post again this is maybe not directly an answer to your question. My html/css skills aren't good so I can't help you with that. But because I think the example I give is a improvement of the code I don't remove it.