Very likely Beamish Salter Benjamin Murdoch (who later became a lawyer and a well known historian), aged 15.B. 1 Aug. 1800, son of Andrew and Elizabeth Ott (Beamish) Murdoch. His mother, an elder sister of Louisa's "friend" Thomas Beamish, died shortly after the birth of her son; and her husband, upon enduring very trying circumstances (see Punch, 1979, p. 274), died only a few years later. As Punch points out, their "only child was raised by the Misses Beamish, maiden sisters of the mother." They would be Sarah Catherine Ott Beamish (see under A25n1) and her younger sisters Harriet and Maria.
The Fergusson typescript of the Louisa Collins diary (made at the Public Archives of Nova Scotia shortly after the diary MS was received in 1953) has "Sally" instead of "Salter"--which would make a lot of sense, she being the elder sister referred to above and her gender being in harmony with the term "the girls." Nevertheless, a close examination of the MS yields "Salter"-or "Satler." See MS .
As for Louisa's use of the term "the girls," it appears that this is an index of her excitement--and of the focus of her attention--upon finally witnessing the arrival of her bosom friend Harriet (see S17n1) and Maria, neither of whom she sees half so often as she sees Thomas.