From 18thcenturyblog.johannaost.com/2011/08/the-tischbein-family#image_1442. This article mentions there were three Tischbeins and that it is difficult to tell which one dis a specific painting. Also posted to Darya Petrovna Saltykova’s new Subalbum here.
I guess that the two oldest daughters, Praskovya (b. 1772), Ekaterina (b. 1776), are shown wearing 80s coiffures and styles. If so, Praskovya is about 13 and Ekaterina is about nine. But they are dressed as mature women.
Keyworlds: 1785, Tischbein, Daria Petrovna Chernysheva, Darya Petrovna Chernysheva, Daria Petrovna Saltykova, Darya Petrovna Saltykova, Chernyshev family, Saltykov family, Lady-in-Waiting, Catherine II, Russian, straight coiffure, floral headdress, fichu, vee waistline, fitted bodice, scoop neckline, lapels, elbow length tight sleeves, cuffs, vee waistline, full skirt, shoes