Hi Elizabeth,
Things have been a bit hectic here recently, but I finally managed to get to the LDS centre to look at the Baptism records for the McGavin family.
They don't really clarify anything, i.e. there's no mention of James being Matthew (!), and it was a good job I already knew who I was looking for, because in a couple of cases McGavin wasn't mentioned in the column for "name of child", i.e. William Thomson and Agnes Boyd - so I could easily have missed them

. No sign of a Jane McGavin either!
Here goes, (in all cases the parents are listed as Robert McGavin and Margaret Skelly):
Janet Skelly McGavin, dob 31.8.1836, baptised 18.9.1836
Mary Reid McGavin, dob 3.7.1838, baptised 29.7.1838
Robert McGavin, dob 3.6.1840 [blank space for baptised date and Minister

]
John A S McGavin, dob 2.7.1843, baptised 26.8.1843
James McGavin, dob 5.12.1846, baptised 3.1.1847
William Thomson [McGavin], dob 21.9.1848, baptised 26.11.1848
Margaret McGavin, dob 27.9.1852, baptised 31.10.1852
Agnes Boyd [McGavin], dob 16.3.1855, baptised 10.6.1855
Will be in touch soon,
Janet