[Disclaimer: This might crush your dream.]
First of all, here are the facts - if I can trust Wiki, that is.
The haiku in question could **_not_** have been written by {}. Why not? That is because Sasaki died in **1955** and the statue was completed in **1958** to commemorate Sasaki and other kids. Sasaki simply could not have written a poem inspired by the statue as she never saw it herself.
Secondly, my gut feeling as a Japanese-speaker more than anything. (That means you can take it or leave it.)
It would be safe to say that the haiku was written by {} though I could not find it anywhere online and **_it was written in English in the first place_**. Thus, there exists no such thing as the Japanese original. Shigemoto seems to have written many haiku in English.