Simple measure of happiness/sadness which has been used in a few Salzburg experiments with children aged 6 and above. (It also works with Austrian professors.) Is it reliable? Is it valid? I’m not sure. But children tend to make the face sadder and happier when you’d expect them to.
Download, open, and set to full screen. Use the mouse wheel to change the smiley/frowny (it should go up and down through pages). Click the button to the right to move to the flat line. The page number can be used to work out the position (26 is the middle, 1 is the happiest, and 51 is the saddest.)