class PersonSerializer(serializers.ModelSerializer):
    children = serializers.SerializerMethodField()

    class Meta:
        model = Person
        fields = ['id', 'first_name', 'last_name', 'mother', 'father', 'children']

    def get_children(self, obj):
        # Get all children of the demo
        children_as_father = obj.children_as_father.values_list('id', flat=True)
        children_as_mother = obj.children_as_mother.values_list('id', flat=True)
        return list(children_as_father) + list(children_as_mother)

https://www.django-rest-framework.org/api-guide/fields/#serializermethodfield