Good news, Grada!
I'd noted down those Denny STI people but decided not to post them

as the names are not uncommon ...
And there's still a need for further confirmation that John HENDERSON & Janet ALLAN who m in Denny STI in 1803 are the right parents rather than John HENDERSON & Janet ALLAN who have marriage entries in Kinnoul and Kinfauns PER in 1812 ... (especially as your Janet has birthplace as Kinnoul in the 1851; and as births for Lavinia and Elizabeth haven't been found).
Just in case it helps anyone (especially Monica who always finds things!) here's what I noted down but didn't post.
John HENDERSON (father John HENDERSON) m Janet ALLAN 1 Apr 1803, Denny STI (there's also an LDS submission giving the place as Falkirk).
Probable children of the Denny couple from the IGI:?
John bap 19 Feb 1804 Denny STI
Janet bap 22 Sep 1805 Denny STI
Margaret bap 14 Aug 1807 Denny STI (d 1857 a/c an LDS submission)
Agnes bap 5 Jun 1811 Denny STI (m John BENNIE a/c an LDS submission)
Christian bap 23 May 1813 Denny STI
Robert bap 3 Sep 1815 Denny STI
James b 15 May 1821 Denny STI
Thomas b 12 Mar 1823 Kilsyth STI (or might he belong to the next couple?)
Probable children of John HENDERSON & Janet ALLAN (who have 1812 marriage entries in Kinnoul/Kinfauns) from the IGI:?
Janet Forrester bap 26 Feb 1813, Forteviot PER
John b 14, bap 21 Aug 1814 Forteviot PER
Alexander b 26 Jun 1816 Rhynd PER
It would be good to find out Margaret (HENDERSON) AITKEN's age; that should be a good indicator ...
From FreeCEN, we find in the 1841 in Cramond Midlothian the following at Dowies Mill:
AITKEN Alex'r 28 Paper Maker
AITKEN Thomas 25 Paper Maker
AITKEN Marg't 26
AITKEN Alison 1
All b Midlothian.
{IGI has, for Alexander AITKEN & Margaret HENDERSON, children Alison bap Oct 1839, Thomas bap May 1843, Elizabeth bap Dec 1845, and Robert bap Sep 1848 - all Cramond, MDL}
So this would give a birth year for Margaret (HENDERSON) AITKEN of 1815 - which suggests she might be a child of the Perthshire, rather than the Stirlingshire, couple? And thus that Elizabeth (Betsey) too is a child of the Perthshire couple ...
More work to be done?
JAP