Margaret
I'm also not 100% on the 1861 family given that the entry is the only one that shows to date and there is nothing else to verify it at present.
Have you managed to find Robert and Elizabeth on
any census after their marriage in 1869 in Dundee? I can see what looks like 3 children born to them in Dundee on IGI:
1. MARGARET THOMSON Birth: 03 APR 1871 Dundee, Angus, Scotland
2. ROBINA THOMSON Birth: 08 OCT 1872 Dundee, Angus, Scotland
3. JAMES ALEXANDER THOMSON Birth: 1875 Dundee, Angus, Scotland/ Death: 21 DEC 1941
However, although you have a possible birth entry in 1842 in Tillicoultry for a Robert Thomson, there is nothing at present to verify that it is your Robert. You would need a census entry to verify Tillicoultry as his birth place. There is a chance that your Robert was in fact born elsewhere which may be why he is not coming up on the index search under that birth place (apart from that 1861 family). For example there is this entry in the 1871 index in Dundee:
Robert Thomson 25, mill worker, b. Ireland
Elizabeth Thomson 24, b. Aberdeen
Address: Crightons Land Hilln, St Clements Dundee
I'm not sure which line you are following up, have you managed to find that child after their birth in the censuses? Always the possibility that either parent had died prior to the 1881 Census.
Struggling to find anything of substance on the censuses
Regards.
Monica