I would have to say that I would think it unlikely that Isaac would have been missed on at least 2 censuses if he was part of the family I found. He should, at least, be on the 1850 and 1860 census for his age. I would think it odd that he would be the only child overlooked.
There was an Isaac Crawford age 2 in 1850 Washington Co., VA listed with a mother Elizabeth age 37 and 2 other Crawford children.No father in the household. I think they may be associated with the family I found though, because Andrew Crawford that is listed with William and Elizabeth in 1850 census that I sent you was in Washington Co., VA in 1840.
In 1880 Isaac and Susanna are in Monroe Co., KY and in Cumberland Co., KY in 1900, 1910.