Hi Dano,
Unfortunately there won't be any more on the OPR's than what you've already got. You're actually quite lucky to have found the entries in both parishes, so well done there.
I looked up the BVR's and found birth entries for your couple's children as follows: (you'll see that there's quite a variation on the spelling of the surnames)
Thomas AITKEN, Birth Date - not listed, Christening Date: 6 Jul 1772
Recorded in: Slamannan, Stirlingshire, Scotland
Father: John AITKEN
Mother: Mary MARSHAL
John AITKIN, Birth Date - not listed, Christening Date: 1773
Recorded in: Slamannan, Stirlingshire, Scotland
Father: John AITKIN
Mother: Mary MARSHAL
Henry AITKIN, Birth Date - not listed, Christening Date: 12 Jun 1774
Recorded in: Slamannan, Stirlingshire, Scotland
Father: John AITKIN
Mother: Mary MARSHAL
John AITKEN, Birth Date - not listed, Christening Date: 3 Nov 1776
Recorded in: Slamannan, Stirlingshire, Scotland
Father: John AITKEN
Mother: Mary MARSHAL
Christian AITKEN, Birth Date - not listed, Christening Date: 11 Jul 1779 Recorded in: Slamannan, Stirlingshire, Scotland
Father: John AITKEN
Mother: Mary MARSHAL
Mary AITKEN, Birth Date - not listed, Christening Date: 30 Dec 1781 Recorded in: Slamannan, Stirlingshire, Scotland
Father: John AITKEN
Mother: Mary MARSHEL
Robert AITKEN, Birth Date: 13 Apr 1784, Christening Date: 25 Apr 1784 Recorded in: Slamannan, Stirlingshire, Scotland
Father: John AITKEN
Mother: Mary MARSHAL
Andrew AITKEN, Birth Date - not listed, Christening Date: 28 Feb 1786 Recorded in: Slamannan, Stirlingshire, Scotland
Father: John AITKEN
Mother: Mary MARSHAL
I had a wee look to see if I could find John's birth c1747 +/- 5 years and there were 5 born in Stirlingshire. The nearest to both Slamannan and Muiravonside would be this one:
John AITKEN, Birth Date - not listed, Christening Date: 30 Sep 1744 Recorded in: Falkirk, Stirlingshire, Scotland
Father: John AITKEN
Mother: Anna FERGUSON
For Mary, this is the only entry again c1747 +/- 5 years. There were 3 born in Stirlingshire but this might be the one you're looking for:
Mary MARSHEL, Birth Date - not listed, Christening Date: 8 Jul 1750
Recorded in: Muiravonside, Stirlingshire, Scotland
Father: Alexander MARSHEL
Mother: Mary RUSSEL
I also looked in the pre-1855 MI Books for any possible Aitken burials but neither Slamannan or Muiravonside churchyards had any listed. This doesn't mean that they're not buried there, it's just that there was no stone found when the graveyards were transcribed back in the 1960/70's.
If you think either of the above (John or Mary) might be likely candidates, get back to me and I can see if they have any siblings listed.
Anne