This little girl, around six years old, and her fifteen year old cousin, would not smile. This happens. I get it. When I was her age I would have run out of the room, being I was very shy, and not liking the whole fairy thing. But that's just me, and some children.
I told the mom afterwards it might have been best had she, and the child's dad, maybe leave. When shooting these sessions and all sessions involving kids, the adults get to sit on one of my couches (I have several, all retro and cool) and watch.
As long as they don't speak, repeat my instructions or coach the kid. That stuff just gets in the way.
However, having all those 'eyes' on her while posing, can be intimidating for sure! We still got some cool images and memories...