Django: ModelMultipleChoiceField multi level

Question

I have used ModelMultipleChoiceField with

class PostForm(forms.ModelForm):
owner = forms.ModelChoiceField(queryset=User.objects.filter(is_staff=True, is_active=True))
groups = forms.ModelMultipleChoiceField(
    queryset=Group.objects.all(),
    required=False,
    widget=forms.CheckboxSelectMultiple())
categories = forms.ModelMultipleChoiceField(
    queryset=Category.objects.filter(is_root=True),
    required=False,
    widget=forms.CheckboxSelectMultiple())

class Meta:
    model = Post
    fields = ('name', 'title', 'url', 'image', 'desc', 'content', 'start', 'pinHome', 'pinNation', 'pinSchool', 'owner', 'poster', 'groups', 'categories')

and my Category model has

class Category(models.Model):
     parents = models.ManyToManyField("self", blank=True, related_name='childs', symmetrical=False)

my template like that

{% for category in form.categories %}
     <label>
         {{category}}
         {% for childs in category.childs.all %}
              {{childs}}
         {% endfor %}
     </label>
 {% endfor %}

How can I access "children" of category on template. I have tried {{category.childs.all}} but it doesn't work.