According to A Dictionary of Scottish Emigrants to Canada before Confederation by Donald Whyte, Donald Robertson was "From Rannoch, PER. To Pictou, NS, on the Sarah, ex Fort William, Jun 1801. Farmer. Wife Janet with him, and ch: 1. Peggy, aged 6; 2. Janet, aged 4; 3. John, aged 2." (bold added by me)
The information for James is the same as the bolded part of Donald's with James' wife and children listed as you have them.
There is also a John Robertson with the same bolded information who was a "Tenant. Wife Janet with him, and ch: 1. Alexander, 2. Elizabeth, 3. Donald, aged 13; 4. Janet, aged 11; 5. Duncan, aged 5."
Jacquie