Sounds like he just got excited, probably knew it was close to the end of the lesson. She could have startled him a little, too, who knows! Horses can have a little spark of energy now and then, it made your daughter a better rider to have that experience. I know whenever my horses that I was riding at that age did something a little funky, I learned something. :)
He is ADORABLE, too! Hope they have a lot of fun together down the road!