Agatha Christie novel, murderer killed child when still a boy, has very small finger on one hand

Hello experts, I read the above book in a German translation a long time ago and would like to know the original title. I've looked through a list of Christie's published novels but none of the titles rings a bell.

It may or may not have Hercule Poirot in it, and I believe the child was killed when tied to a tree while playing cowboys.

Thanks for your help.

Edited for typo.

Hi!
I've done a search for agatha christie+disturbed children on google. Could it be "Towards zero"? I'm certain that I've read the same book as you :)
"The dinner party is not so comfortable, as Lady Tressilian had predicted. That night, Mr Treves told a story of an old case, where a child killed another child with an arrow, which was ruled an accident. The child was given a new name and a fresh start, despite a local man having seen the child practising assiduously with a bow and arrow. Mr Treves remembers the case and the child as a result of a distinctive physical feature that he does not describe. " (https://en.wikipedia.org/wiki/Towards_Zero)

I've just looked at the Wikipedia page and the information LT holds on this title, and I'm positive this is the book I've read. Thank you so much for your help!

Enjoy the est of your weekend.

Book found: Towards Zero by Agatha Christie

P.S.: For anyone interested, based on the information kindly provided by taafw, I believe the German title of the book is Kurz vor Mitternacht, which translates as "Just before Midnight".