I think it depends on the child. This does seem fishy to me, and might end up being a cover-up because the parent knows that a 5 year old wouldn't get the same punishment as an adult, or even the 12? year old.
But yes, like a few others in this thread, I absolutely knew what I was doing at 5 regarding death for humans and pets. Now, 37 years later, children are maturing at a faster rate. So I completely believe it's possible that the child knew exactly what drowning the toddler would do.
For those who like to research, there's a private hospital near Chicago - or was - that had two doctors performing hair testing studies on serial killers and applying it to sociopathic or out of control children. It was found that in many cases, all it took was a personalized supplement regimen of the exact minerals (and maybe vitamins, too) that the child was lacking to change the child's behavior in a positive way. Even minute amounts made a difference. They also studied various known serial killer's hair and found that each was different, but if memory serves me, they all had either a zinc or niacin deficiency. (Forgive my lack of details. This was on Maury Povich probably 15+ years ago.) I don't know if it were the same people, but there also was a study done linking hypersensitivity to mercury with causing just enough brain damage to change the behavior of the child to be more violence-prone.
Anyway, everyone is entitled to their own opinions. I hope that those who don't believe a 5 year old could do something like kill a toddler will keep an open mind. Check out CrimeLibrary.com for many case examples; although, I think the youngest case I remember was a 6 year old. It's a sad subject.