It's all in your interpretation of the definition, I guess. I have a friend of the family that drove drunk one night with 2 friends in the car. They hit a telephone pole and one of the passengers was killed. The driver, (who was over the allowed level for alcohol in his system) was given 6 years in jail for aggravated manslaughter. It was an accident but due to negligence.
If you have a pool and a 2.5 yr. old. You're on the computer or you've fallen asleep and you didn't check to make sure the doors were locked and the child was inside with you before you went on with whatever you wanted to do, then yes, you are negligent because you didn't take all precautions to ensure the safety of your child.
It would be the same if they were in the backyard already and she got on the phone, not paying attention to what the baby was doing and Caylee climbed that ladder and she drowned. YOU can be held responsible because you did not ensure the safety of your child.
She could have been held legally responsible. Lots of families that have lost children this way were not prosecuted and the drowning deemed "accidental". If this is "culpable negligence" and if Florida courts were to exercise the law as it is written, there would be a lot more people to face the charges. Many don't get to the courts and are written up in police reports as an "accident".
That being said, I don't believe that's what happened here. Too many of FCA's activities in March that indicate to me this was planned, far in advance and so the point of the alleged "accidental" drowning is moot IMHO.