Isn't the girl in the Roman Mysteries an only child?
I think that a lot of mystery novels have an only child - for some reason, in this type of mystery novel, only children are often portrayed as being more intelligent and thoughtful than children with siblings and this seems to make them excellent detectives.
Jupiter Jones in The Three Investigator series is an only child, but also seems to be an orphan. There's an only child called Dan with a dog called Baskerville in another set of detective stories for children, but I don't remember the title of the series.
I think that Harriet in "Harriet and the Cherry Pie" is an only child.
I second the recommendation of "The Good Master" by Kate Seredy - a lovely book about the city girl coming to her cousins in the country.
Thinking of Diana Wynne Jones again, Christopher in "The Lives of Christopher Chant" is an only child, and I think that Marianne in "The Pinhoe Egg" is too (but there may be a much older brother or sister I've forgotten). Janet in "Charmed Life" is an only child, and I think that all three of the children in "A Tale of Time City" are only children. Even when Diana Wynne Jones's characters have brothers and sisters, they tend to be much much older, or not available for some reason. Everyone in "Witch Week" appears to be an only child, which for an entire school, is quite an achievement.
Steve Alton has two only children in 'The Malifex'.
A more miserable example is 'When Marnie Was There' another 80s book being right on, in which the abandoned and traumatised Anna meets the traumatised and abused Marnie, who turns out to be the ghost of her mother, and Anna realises why she was abandoned. Probably best avoided.
Coming back to the question of why are only children in books often orphans or have lost a parent - at a discussion among 4 leading childrens' authors I attended, they agreed that the first thing you have to do in a children's book is 'get rid of the parents'. Until you get rid of the parents, the young reader can't really believe that the characters are allowed to do the things they do, or that they need to do the things they do. So this can be by killing off the parents, making them extraordinarily vague (e.g. Enid Blyton), or having them frozen or otherwise incapacitated by a spell which the child then has to unfreeze (e.g. Steve Alton, Diana Wynne Jones). Another way to do it is to substitute parents with uncles and aunts, who for some reason are always portrayed as vague and easy to trick (due to not being innately aware of what the children are up to, which parents obviously are considered to be) (e.g. Tom's Midnight Garden).