AutoMapper pass parent reference when mapping child object

It turns out that the answer was right there all along. The UseDestinationValue option does exactly what I want.

This options instructs AutoMapper to use the existing property on the target object and map any child properties or collection items into that object rather than creating a new proxy object.

So, here's all I have to do in my application:

Mapper.CreateMap<ParentDTO, ParentObject>()
    .ForMember(obj => obj.Children,
           opt.UseDestinationValue());

And, voila! Now I can instantiate the child collection, with parent reference, and setup the reference back to the parent in each item as it is added to the collection.