The child wasn't Samuel's
Why don't you think he was the father? The part about her being his widow may not have been true but he could still have fathered her child.
Because Samuel was not named as the father on the birth certificate - he space for the father's name is blank.
The 1881 marriage to Charles Thorpe describes her as a widow. There are some details of the marriage on ancestry you may have now seen.
Thank you. I don't subscribe to Ancestry.
As an aside I wonder what they were all doing in Scotland when born in London 
Charles and Beatrice and her parents I mean.
Beatrice's parents were both born in Scotland. They were married in Scotland and their eldest child was born in Scotland. In 1841 James Buchanan was a tailor's journeyman. In 1851 he is described as a sergeant in the Metropolitan Police. In 1861, when they were in Woolwich, he is described as 'corporal military man'. I have yet to find him in the 1871, but in 1881 he was living in Glasgow, and described as a tailor. His wife Margaret, born in Edinburgh, died in Glasgow in 1891, and James himself, born in Ayr, died in Glasgow later the same year. So it looks like a classic case of Scots going to London for employment, and returning to Scotland later in life.
What is more curious is what Charles Chaplin was doing In Scotland. Perhaps he just went there long enough to marry Beatrice, whom he had known in the south-east of England.
There is a Samuel Mclure born 1849 in Edinburgh in 1861.
Yes. He is in the home for deaf and dumb boys, having lost his hearing by a fall according to the census, which gives his place of birth as Dreghorn, Ayrshire. A Samuel McClure, aged 13, mother's maiden surname Caughy, died in Dreghorn in 1861. His parents match the 2-year-old Samuel McClure living in Dalmellington in Ayrshire in 1851, born in New Monkland, Lanarkshire.
There is no other death in the Scottish deaths index of a Samuel M*c*lu*re* who could by any stretch of the most acrobatic imagination be Beatrice's husband, unless she was lying and she wasn't a widow at all.
