I think his point is that he won't show a child getting brutally murdered on-screen, which is fair enough in my opinion. Leone went as far as he could. Art does kill a lot of kids in this movie, but he never shows the actual act on-screen, just the aftermath.
Yes, it's one of the best films I've watched in a while. That said, 2024 has been a really good year for indie horror. The Substance is really incredible also! Well worth watching.