This is my kind of humour too, it's silly, creative, and in spite of it's surface crudeness can oddly be a strong way of communicating your true feelings.

It's probably best left unspoken but my interpretation is that it's likely the little girl was initially being sincerely spiteful, but then being intimately spiteful can sometimes backfire with empathy, gradually transforming her attacks into this loving joke. I actually prefer that interpretation because it shows the capacity for change, but I might be seeing things that aren't there... His interpretation of her being so insightful from the start is also a nice interpretation, consciously or not.