Strange that Thomas's birth and Robert and Margaret's marriage is not on the IGI. And unfortunate as Robert and Margaret's marriage would give their father's names which would make it possible to find them in the birth records and the earlier censuses.
Robert Henry's birth is listed at
www.lawsons.ca:
KINNISH, Robert Henry 4 Nov 1889 S San 361 KINNISH, Robert MOORE, Margaret
I think you will also find record of Robert Henry in the marriage and burial indexes on the same site.
Robert, Margaret and Thomas are in the 1881 IOM census (see
www.familysearch.org). You don't indicate how much or how little you already know about this family, or whether you have access to the censuses but given you have their approximate dates of birth I am guessing you have also already seen the 1891 census. Let me know if you haven't and I will transcribe it for you.
So from the censuses we have Robert born approx 1852-53 in Braddan, Margaret born approx 1855-56 in Malew (both Braddan and Malew are neighbouring parishes to Santan, see
http://www.isle-of-man.com/interests/genealogy/). Closest match on the IGI for Robert is ROBERT KENNISH, Christening: 24 NOV 1850 Santon, Isle Of Man, Father: CHRISTOPHER KENNISH, Mother: ALICE COWIN. No specific entry for Margaret stands out amongst the possibles.
There is a marriage between Robert Kinnish and Margaret Woodworth in Santan 28 MAR 1876. But no resulting children listed. Also only matching birth for Margaret Woodworth is in 1858 in Rushen. Very confusing.
Hope I haven't just made things worse.
Ruth