20/20 hindsight? Or a Lick of Common Sense/Reasonable Person?
If an adult puts 18 mo old on chest-high, rounded rail, either standing or sitting, and fails to keep a firm grip on tot, to prevent a 36 - 46 inch fall to floor, just takes a bit of common sense to understand a tot falling from that short height could incur serious injury, e.g., broken arm, fractured femur, broken spine, traumatic brain injury.
And if adult stands close to window, while putting tot on that rail (or window sill), and the (potential) drop is 100+ feet, result of fall is not going to be 'just' serious injury.
If g'pa did not have that much common sense,
or
if he decided to ignore those ^ risks,
he was negligent.
(20//20 hindsight not needed)
And his negligence caused Chloe's death.
Negligent Homicide. Guilty. jmo.