Well, since you offered to help...

model.py

from django.db import models

class Parent(models.Model):
    name = models.CharField()

class Category(models.Model):
    "Each Parent object has its own set of categories"
    parent = models.ForeignKey(Parent, related_name="categories")
    name = models.CharField()

class Item(models.Model):
    """
    Each Parent object has its own set of items,
    which can be assigned to categories,
    but only categories that are also assigned to the same Parent.
    """
    parent = models.ForeignKey(Parent, related_name="items")
    category = models.ForeignKey(Category, related_name="items")
    name = models.CharField()

# The problem is that autogenerated form fields,
# including the Django admin pages and Wagtail (https://wagtail.io)
# include *all* possible categories in the Item.category queryset,
# instead of *only* those categories scoped to the item's parent.
# I thought I might be able to do something like this:
#
#     category = models.ForeignKey(Category, related_name="items",
#                       limit_choices_to=models.Q(category__parent=models.F('category'))
#                )
#
# But that doesn't appear to work either, since `limit_choices_to`
# seems to operate from the perspective of Category, not Item. Help?
    

You'll need to override the ModelForm's __init__ method, and change the queryset there.