Django CMS plugin nesting --- child doesn't show up in the "structure" interface - django-cms

Question

I'm trying to create a Django CMS custom plugin that can assemble other plugins.
As far as I can tell, Django CMS can do this using Plugin nesting, and I've followed the examples to create a simple test case.
My expectation is that when you go into the "Structure" tab for a record in the model that has a PlaceholderField that includes the parent plugin, when you add a parent plugin, the pop-up for that model should ALSO have some way to edit/create/add an instance of the child plugin. But it doesn't --- all I see are the fields for the parent plugin and NOTHING about the children (see screenshot below).
Or am I missing the point of Plugin nesting entirely?
models.py:
from django.db import models
from cms.models import CMSPlugin
from cms.models.fields import PlaceholderField
from djangocms_text_ckeditor.models import AbstractText
class CustomPlugin(CMSPlugin):
title = models.CharField('Title', max_length=200, null=False)
placeholder_items = PlaceholderField ('custom-content')
renderer = models.CharField('Renderer', max_length=50, null=True, blank=True,
help_text='This is just to show that a custom renderer CAN be done here!')
class ChildTextPlugin(AbstractText):
pass
cms_plugins.py:
from cms.plugin_base import CMSPluginBase
from cms.plugin_pool import plugin_pool
from django.utils.translation import ugettext as _
from .models import CustomPlugin, ChildTextPlugin
class CMSCustomPlugin(CMSPluginBase):
model = CustomPlugin
name = _('Custom Plugin')
render_template = 'custom/custom_plugin.html'
allow_children = True
def render(self, context, instance, placeholder):
context = super(CMSCustomPlugin, self).render(context, instance, placeholder)
return context
class CMSChildTextPlugin(CMSPluginBase):
model = ChildTextPlugin
name = _('Child Text Plugin')
render_template = 'custom/child_text_plugin.html'
parent_classes = ['CMSCustomPlugin',]
def render(self, context, instance, placeholder):
context = super(ChildTextPlugin, self).render(context, instance, placeholder)
return context
plugin_pool.register_plugin(CMSCustomPlugin)
plugin_pool.register_plugin(CMSChildTextPlugin)

Answer 1

... and the answer is "it was working all the time" --- the interface comes AFTER the screen I posted above is submitted --- the Custom Plugin entry will have a "+" icon, and it's THERE that the children are found.