Infanta Carlota Joaquina was just ten when this rather unflattering portrait of her was made by Maella in 1785, the year of her official marriage. Her Wikipedia article is here.
Carlota Joaquina is dressed in a variant of the traditional gurdeinfante - the tontillo. Her dress has a lace flounce, much like those of the Countess of Provence and the Princess Lamballe. The gigantic panniers support a train. She wears a scoop neckline and a high coiffure under a wide cap as seen on Mme. de Genlis.