John Trevelyan, 4th Baronet's, Wikipedia article is here and his article in the peerage is here. Louisa's article in thepeerage is here. John's article in thepeerage does not list all of his children, but does state that the teenage son was born in 1761.
From bbc.co.
The style of painting suggests pre-1750 as do the short coiffures. This shows 1770s casual style included tight sleeves for the young and slightly rounder sleeves for adults. The dresses appear to be round gowns with no sign of waffling panniers.
Keywords: Devis, Louisa Simond, Louisa Trevelyan, Helena Trevelyan, British, straight coiffure, hat, chemise, high neckline, collar, bow tie, long close under-sleeves, jacket, buttons, long tight sleeves, cuffs, peplum, full skirt