I should have looked at this before, but here are a few observations on children in preparation for animating my little girl character.
To start with, I googled child actresses and liked this pic Lennon Wynn. It follows on nicely from the open/closed mouth post. The child's closed smile (together with the sparkle in her eyes) indicates a knowing-ness. She is stood up right with a confident posture.
Next, is this clip from Miracle on 34th Street. I love the patience of Santa Claus, then how big and bright the happy surprise meets her eyes.
A last note for now, is on this shyness of the boy when asking this question that is bothering him. The downward check of the eyes, to double check with him if he should ask or not.