Are you looking at the original census, or at a transcription?
The index at Scotland's People says Samuel was 39 in 1881, but another transcription says 34. He is in a household with his brother Bernard, also aged 39, so I think it would be worth a look at the original to see who has got it wrong.
But there isn't a 3 or 4 year-old daughter in the household. Their only child is son Samuel, aged 1, born England.
There are two Samuel Bradys born in England in 1880, one with mother Snelson and the other with mother Smith.
James Brady married Elizabeth Snelson in Liverpool in 1863, and they had a large family. Samuel b 1880 died in infancy. His father James also died about the same time, and Jane Brady, widow, is with her mother Jane Snelson in Ptrescot in 1881. So they cane be discounted.
The one with mother Smith was born in West Ham. He had an older brother Charles, b 1877, and this allowed me to find the family living in West Ham in 1881. So they too can be discounted.
There is a William Brady, mother Carroll, born in Liverpool in 1880. But this isn't helpful either because he had an older sister Susannah, and both are with their mother Mary Ann and siblings, not including a Samuel, in Liverpool in 1881 (well, West Derby) in 1881.
Bernard and Margaret's son Alexander is aged 1, born Lanarkshire. He would fit the one born in Carluke in 1880, mother Heenan. I think that this could be Bernard Brady and Margaret Keenan, who were married in Longford, Ireland, in 1870. Bernard's father was also Bernard, and their daughter Mary, born in Longford in 1872, would fit the 9-year-old Mary in the 1881 census.
Samuel Brady, son of Bernard Brady, married Jane Murphy in Dublin in 1873. It's the only marriage on record of a Jane to a Samuel Murphy in Ireland before 1881 but I still can't see a birth of a Samuel Brady in England in 1879/1880 with mother Murphy.
I don't see Samuel and Jane in 1891 or later census in Great Britain, even with spelling variants, and I don't see them in Ireland in 1901. Maybe they emigrated somewhere?
What were the name of the daughter of Samuel Brady and Jane Carroll, and the name of her husband? Have you found them in the 1901 census, and if so where does it say that she was born? And how old was she? Have you found her in the 1881 census?
