Do you have access to any studio lighting?, The on camera flash gives a head-on spread of light and gives harsh shadows. If you can get your hands on a studio light with a soft box, try lighting from different different angles, it should give you more interesting lighting and softer shadows.
Subject is too central when using Landscape format for a portrait. Try putting them to one side of the frame ie rule of thirds or else use portrait format and get in tighter.
I agree with the comments on the positioning of your subject & on camera flash. There is too much room on either side of your subject. The harsh shadows caused by on camera flash also ruin the shot. You have done well in avoiding reflections in the girls glasses which can be a big problem using your flash on camera too. If you don’t have an alternative way of firing your flash, try using natural light, but avoid direct sunlight. I don’t shoot portraits, (too scary) :lol: so you can take my comments with a grain of salt. :)