These are the children showing to an Alexander Turnbull and Janet Masterton, inlcuding Alexander that we already had, all from IGI:
1. JOHN TURNBULChristening: 06 JUL 1828 Ceres, Fife
2. ROBERT TRUMBLE Christening: 30 MAY 1830 Ceres, Fife
3. ALEXANDER TURNBULL Christening: 08 APR 1832 Ceres, Fife
4. JANET TURNBULL Christening: 06 JUL 1834 Dunino, Fife
5. WILLIAM TURNBULL Christening: 12 JUN 1835 Dunino, Fife
6. ELIZABETH TURNBULL Christening: 18 DEC 1841 Carnbee, Fife
I thought this might be the family in the 1851 census:
Alexander Turnbull 44, Ag. Lab. & Farmer Of 5 Acres, b. Kemback Fife
Janet Turnbull 50 b. Forfar, Forfarshire
William Turnbull 14 b. Denino, Fifeshire
James Turnbull 11 b. Carnbee, Fifeshire
Elizabeth Turnbull 9 b. Carnbee, Fifeshire
David Turnbull 5 b. Carnbee, Fifeshire
Address: Overbarnbee, Carnbee, Fife
From this info, I wondered whether this might be Alexander Snr.'s entry on IGI:
ALEXANDER TURNBULL Christening: 30 DEC 1806 Kemback, Fife
Parents: JOHN TURNBULL and ELISABETH JACK
Monica

Added: Likely entry for 1841, with the surname mistranscribed on the census I am looking at;
Alexr Tumbul 35, agr. lab.
Janet Tumbul 40 b. Scotland
Robert Tumbul 11
Alexander Tumbul 9
Janet Tumbul 6
William Tumbul 5
James Tumbul 1
Address: Bonerbo, Carnbee Fife