I think I have found them in the 1841 census on Freecen.
Crews, Old Luce, Wigtownshire
Samuel McClelland 25
Grace McClelland 20
John McClelland 3
David McClelland 1
This fits with the Extracted record for Christening listed on IGI
John Mclellan 18 APR 1838 Old Luce
There is another son (Andrew) born in 1855. If you view this certificate it should give a lot more information, such as number of siblings, whether dead or alive...