It had gone so well up to that point. She'd smiled, she'd posed, she'd followed just about every instruction above and beyond what had been expected of her. She had been such a good girl.
So her mind was elsewhere. Focused on getting pictures done. Focused on being such a good girl, that she just simply forgot. She was only small after all, and this whole "big girl underwear" concept was fairly new. She just forgot, that's all. She wanted to be such a good girl.
And that is how the final shot of the day showed the slightly guilty, half-surprised, knowing look of a young lady that had...
Just forgot.
But her daddy just laughed and gave her a hug and told her it was all right, and that she was a very good girl.