I would guess abdominal trauma, possibly causing a ruptured spleen and/or liver, that will cause a major intra-abdominal bleeding, and result in shock and death. Abdominal trauma - WikipediaHow do you kill a child with a belt? The buckle to the head?
There war a case here in Sweden where a little boy was hit in the stomach, and wasn't taken to hospital for several hours, and when he got there it was too late, he died of the intra-abdominal bleeding.