Agnes Ryder, other surname Smith, died at Middlebie in 1955 aged 78. If that age is correct, she was born 1876/1877 (and Andrew Cameron Smith was born 1874/1875 because he was aged 68 when he died in 1943).
Agnes Jane Smith married John A Ryder in Kirkmichael, Dumfries-shire, in 1910.
There's a birth of an Agnes Jane Smith, mother's surname Cameron, in Kirkmahoe in 1878.
In the 1881 census there is a family in Kirkmahoe including George Smith, 34; Helen Smith, 34; Andrew Smith, 7; James Smith, 6; and Agnes Jane Smith, 3.
In 1891 there are George Smith, 44; Helen Smith, 44; Agnes Smith, 13; George Smith, 9; Helen Smith, 6; and Jane Smith, 4.
In 1901 there are George Smith, 54; George M Smith, 19; Agnes Jane Smith, 22; and Nellie C Smith, 16.
In the birth indexes for Kirkmahoe are the following Smith children with mother's maiden surname Cameron:
Agnes Jamieson, 1876 - this child died in infancy
Agnes Jane, 1878
George Morton, 1881
Helen Cameron, 1884
Janie Morton, 1887, died aged 12 in 1900
Two older children were born in Loudoun, Ayrshire
Andrew, 1873
James, 1875 - this child died in Kirkmahoe in 1884, aged 9
George Smith and Helen Cameron were married in Galston, Ayrshire, in 1872.
Helen Cameron or Smith, mother's maiden surname Jamieson, died in Kirkmahoe in 1895, aged 49.
So I think that your Mr Smith and Mrs Rider are Andrew and Agnes Jane, children of George Smith and Helen Cameron.
You need to take a look at those two death certificates and see if I'm right. Andrew's will confirm the cause of his death.
I also note with interest that George Morton Smith, mother's maiden surname Morton, died in Dumfries in 1933 aged 87. This is an exact match for the age of George Smith senior in all those censuses, so I think he's likely to be the widower of Helen Cameron and father of Andrew and Agnes Jane. Again, his death certificate will clarify that.
Over to you - let us know how you get on.