Kath,
Without the full entry it is difficult but at a guess I would say James Stevenston was son to Agnes before she married Robert
Fleming. However in the 1851 Index he is shown as James
Fleming 7y, with William 5y, George 3y, Robert 1y and William Stevenston aged 18. Ed 2 page 19
When transcribing other censuses for Freecens I have found quite a number of errors in the Index so I suggest you need to check the full census just in case James's middle name was Stevenston.
Regards
PS: The 1861 Index shows birthplaces for Agnes and all the children as Kilmarnock whereas Robert
Fleming was born in Perth Kincadam? may be Kincarden??
Duncan.