category subcategory not showing properly drf django

i have this category models:

class Category(MPTTModel):
      name = models.CharField(max_length = 100)
      slug = models.SlugField()   
      parent = TreeForeignKey('self',blank=True, null=True, related_name='children', on_delete=models.CASCADE, db_index = True)


     class MPTTMeta:
           unique_together = ('slug', 'parent') 
           verbose_name_plural = "categories"
           order_insertion_by = ['name']

    def __str__(self):  
        full_path = [self.name]       
        k = self.parent       
        while k is not None:
              full_path.append(k.name)
               k = k.parent
        return ' -> '.join(full_path[::-1])

serializers.py:

category Listing Api

class CategorySerializers(serializers.ModelSerializer):
      class Meta:
            model = Category
            fields = ('id','name','slug','children')

      def get_fields(self):
          fields = super().get_fields()
          fields['children'] = CategorySerializer(many=True, read_only=True)
         return fields

views.py:

class CategoryView(generics.ListCreateAPIView):
      queryset = Category.objects.filter(parent__isnull=True)
      serializer_class = CategorySerializers

currently its working but i want to show subcategory inside category only not as a main category as you can see subcategory are listed inside main category and with maincategory also

I am expecting this result:

   [
   {
        "id": 1,
        "name": "Animals & Wildlife",
        "slug": "animals_wildlife",
        "children": []
   },
   {
        "id": 2,
        "name": "main cat",
        "slug": "mai_cat",
        "children": [
        {
            "id": 3,
            "name": "sub_cat",
            "slug": "submaincat",
            "children": []
        }
    ]
   },

{
    "id": 4,
    "name": "maincat2",
    "slug": "maincat2",
    "children": [
        {
            "id": 5,
            "name": "submaincat2",
            "slug": "submaincat2",
            "children": []
        }
    ]
},

{
    "id": 6,
    "name": "maincat3",
    "slug": "maincat3",
    "children": []
}
]

...............................................

Change your viewset to

 class CategoryViewSet(ModelViewSet):
     queryset = Category.objects.filter(parent__isnull=True)
     serializer_class = CategorySerializer

and serializer to

 class CategorySerializer(serializers.ModelSerializer):

     class Meta:
         model = Category
         fields = ('id', 'name', 'slug')

     def get_fields(self):
         fields = super().get_fields()
         fields['children'] = CategorySerializer(many=True, read_only=True)
         return fields