Being "... clearly old enough to understand what they were doing" is irrelevant to the distinction between murder and manslaughter.
To get a murder conviction you have to convince a jury that the defendant's actions were taken with the express intention of doing harm so serious it would end the victim's life.
It is extremely unlikely that this boy set out with the intention to kill the man that day, and even less likely that a jury would be convinced of that.
The victim suffered catastrophic injuries largely because he was incredibly frail, rather than purely due to the severity/duration of the attack. The boy kicked him, punched him to the ground and hit him with his flip-flop.
If he'd have kicked his head around like a football, stamped on his neck, stabbed him in the chest etc, it may have been different. But the reality is that it's often difficult to prove pre-meditation (as it should be) and as a result, manslaughter is seen as a much safe route when there's doubt involved.