The boys were 9 and 4, so I guess the 9 yr old must have gone to school. I think the boy was probably so traumatized he didn’t know what to do and he probably thought he was protecting his younger brother until he got to some place where he knew adults would help him.
It’s also possible that the attacker told the boys not to leave the house or call for help. The father was also at home, but he had been beaten in the head and left for dead.
The boy thought his father did it, so there was a protective element in his delay to say anything (nark on his dad).