How to access and style singel html-tag within html.twig

i am trying to use bootstrap within html.twig-file. Situation: i have this code line within html.twig:

{{ form_row(form.gender) }}

Here is a definition of "form.gender" within php

 ->add('gender', ChoiceType::class, array(
                'choices' => array(
                    'form.register.children.gender.choice.male' => 'm',
                    'form.register.children.gender.choice.female' => 'f'
                ),
                'multiple' => true,
                'expanded' => true,
            ))

From this code-line will be generated big html-area:

<div>
  <label class="required">Gender:</label>
  <div id="new_entry_gender">
    <input id="new_entry_gender_0"                
           name="catalog_new_entry[gender][]"      
           value="m" 
           type="checkbox">
    <label for="catalog_new_entry_gender_0">Male</label>


    <input id="new_entry_gender_1" 
           name="catalog_new_entry[gender][]"     
           value="f" 
           type="checkbox">
    <label for="catalog_new_entry_gender_1">Female</label>
  </div>
</div>

I would like use bootstrap-classes for any singel html-tag like: label, input But i have only one line:

{{ form_row(form.gender) }}

How can i access to any singel html-tag (for styling), when i have only one line above? Thx Thanks