What's the best way to create objects with multiple children in code?

[graysham]

I am attempting to adjust code I have already written to work with polymorphic-tree and am failing to get it working.

Base setup is

ParentModel(PolymorphicMPTTModel):
parent = PolymorphicTreeForeignKey('self', blank=True, null=True, related_name='children', verbose_name=_('parent'),
on_delete=models.CASCADE
)
some_date = models.TextField(null=True)

ChildModel(ParentModel):
child_data = blah

Grandchild1Model(ChildModel):
data = blah

Grandchild2Model(ChildModel):
data = blah

I have tried something along the lines of

d, created = ParentModel.objects.update_or_create(some_date=blah)
d2, create = ChildModel.objects.update_or_create(child_data=blah, parent = d)

This however does not seem to work. Any tips would be greatly appreciated.