A view of Lady Theresa Susey Helen Stewart, Marchioness of Londonderry with Charles Stewart (1878-1949), (became 7th Marquess of Londonderry) and a girl in the grounds of Wynyard Park c1890. 
Picture Stockton Archive
A Pictorial History of Stockton-on-Tees
The little girl is probably Helen Mary Theresa (1876-1956), daughter of Theresa, Lady Londonderry. Helen married the 6th earl of Ilchester.