Possible to theme the form's elements with own twig?
I found out a solution which I ended with using {{ element.[machine name] }} instead of {{ children }} on my custom template which is cloned by form.html.twig. The result of the code look similar below:

    &lt;div class="row"&gt;
        &lt;div class="col-md-6"&gt;
            {{ element.name }}
            {{ element.mail }}
            {{ element.subject }}
        &lt;/div&gt;
        &lt;div class="col-md-6"&gt;
            {{ element.body }}
        &lt;/div&gt;
    &lt;/div&gt;
    &lt;div class="row"&gt;
        &lt;div class="col-md-12"&gt;
            {{ element.submit }}
        &lt;/div&gt;
    &lt;/div&gt;


If you have other suggestion, I am happy to know! :)