I was checking your posting and notice I have record of a son David born to this couple, christened 26 July 1815. I have a birth for a James Mill/Milne at Monikie c20 Feb 1779, son of James Milne and Helen Ker which may be a possibility for you. This couple also had a son William c 30 July 1783.
My family is James Mill and Margaret Baird whose youngest child, Alexander was born at Monikie in 1813.