Thomas' baptism is recorded in FamilySearch as Scotland Births and Baptisms, 1564-1950
Indexing Project (Batch) Number C11485-2
System Origin Scotland-VR
GS Film number 1041953
Reference ID 2:17VP3BN
I then ordered the films from LDS library and photographed the record.
The name stays McKen with David & Margret Laing, then their son Thomas (1766) & Mary Brown, then their son David (1795) and Janet Rankin, married 1815, Stirling. It changes with David's son, James Rankin McKean (b. 1839, Denny) marries Janet Hill (1864, Glasgow).
I listed the children of the 3 generations: David & Margret, Thomas & Mary, and David and Janet in #8.