Robert McKee died intestate in Chester County, so no children were listed. The info about the unnamed children comes from the will of Gayen Moore, proved in Philadelphia County. I have only seen a transcription and an abstract of the will. It mentions Robert's daughter Sarah and four children. Whether or not that would be four other children or four children including Sarah, I don't know. But he had two sons, John and Isaac, by his second wife, who was a daughter of a Quaker named John Todd. He must have raised them and they stayed in New London (I think John was an officer in the militia during the Revolution; don't know what happened to Isaac).
I feel pretty sure that Gavin had brothers William (d. 1823) and Robert (d. 1828). Robert (d. 1755) could be their father, but it's iffy, because most likely Robert would have married his second wife by 1752, in order to have two children.But in a lot of ways he seems to fit the bill (the name Robert, his date of death, the connection to a Moore family).