They had three children that we know of:
She may be the Helen Smith who was born in Torryburn on the 12th May 1776 to Thomas Smith and Isobel Anderson, certainly she was married in Torryburn which might indicate she was born (or at least living) there. This is the only record of a Helen Smith born in the parish of Torryburn in the correct age range, and it would mean that she was 29 when she married John Balfour, and 35 when she had her son James Balfour.
In the 1841 census she is living in the house (in Torryburn High Street) of her son James Balfour, along with her other son David Balfour. In the 1841 census the ages were rounded down to 5 years, her age was given as 65 (so was between 65 and 69) so her year of birth was between 1772 to 1776).
She died on the 23rd June 1860 in Torryburn according to her entry in the register of deaths. Her age was given as 85. She was buried in Torryburn churchyard. Her parents were given as Thomas Smith (Master slater) and Ann Anderson. I cannot find any records of an Ann Anderson marrying a Thomas Smith, so this is more likely to be another name (or mistake) for Isobel Anderson.
GROS (ScotlandsPeople): Birth/Baptism 12/05/1776 Helen Smith (to Thomas Smith, Isobel Anderson), Torryburn GROS 458/00 0002
GROS (ScotlandsPeople): 26/10/1805 John Balfour married Helen Smith, Torryburn GROS 458/00 0002
GROS (ScotlandsPeople): Birth/Baptism 15/05/1811 James Balfour (to John Balfour, Helen Smith), Torryburn GROS 458/00 0002
GROS (ScotlandsPeople): Birth/Baptism 15/06/1817 David Balfour (to John Balfour, Helen Smith), Torryburn GROS 458/00 0002