Hi Suz
The 1st daughter...was actually born abt 3 years before John & Agnes married? Perhaps she wasn't the natural child of John - the 1841 Census tends to indicate as much?
1841: Philps Close, Dysart
John McGILVARY 20, Flax Dresser
Agnes McGILVARY 30,
Agnes DUNCAN 4
If we only look at the children born AFTER the marriage we get a naming pattern to consider:
Isabella (1st daughter)
John (1st Son)
James (2nd Son)
Mary (2nd daughter)
Which very nicely fits the traditional naming pattern, per the two older persons being baptised in 1852………that is:
1st dau & 2nd son - parents of Wife
1st son & 2nd dau - parents of Husband
I do agree those 2 late-life baptisms are very likely to be your John & Agnes, getting themselves all ready to immigrate - perhaps some official proof of their birth being required.
And I still think that - even though there 'healthy' gaps in which other children could have been born, who may have died soon after birth, or pregnancies which did not go to term:
between 1 & 2 (2yrs, 3mths)
between 3 & 4 (5 yrs 6mths)
Looking at the 1851 Census (FREECEN) Agnes is 36 (born abt 1815) so no conflict there with the baptism stated birthdate. John is age 30 in the Census (born abt 1821), which is abt 4 yrs 'out', not too wildly out - what dates etc do you have for him in Australian records?
Cheers
AMBLY