Isn't the bolded exactly what a court case would determine? I don't think anyone is saying GF had any kind of intent of this accident happening. That doesn't mean he wasn't negligent.
Potentially it could be a question of fact for a jury in a trial.
In order to bring the charge the Prosecution should be able to establish probable cause that he either did know, should have known (a reasonable person should have known), or had the capability to know that the risk existed. That's the type of thing that will likely be the lynchpin of the argument at a preliminary exam, where a court will decide if enough probable cause has been shown to allow the case to proceed. It seems likely to me that at that stage a deal is likely to be struck. (Just my opinion) I think a lot of the arguments in the case would eventually boil down to, how familiar was grandpa with the situation and surroundings, as in did he grasp the scope of the location and the physical circumstances? It has to be considered in the shoes of someone of his experience, his age and his physical abilities, along with any distractions at that time that might have caused him to overlook factors that raised the danger. It cannot simply be viewed in the light of, I've been on that ship/ships like this before and it's apparent to anyone. You or I might walk into that location and immediately recognize that what he did was out of line, even if we've never been there before.
Are there any circumstances that you or I would ordinarily walk in and understand the risk, yet another time be in a situation where the immediate distractions of the surroundings/circumstances of travel and/or any potential medical conditions that have been thrown out of whack by changes in diet, routine, rest, etc. cause us to miss the cues that there is great risk here or that I can't handle a squirming toddler quite as much as I assumed because I didn't recognize my stamina, balance or focus are not up to par? Maybe you or I are just enough younger, fitter, healthier that we wouldn't be distracted, thrown off kilter by the circumstances and make the same choices he did, but does that mean it's impossible for everyone else? Is it absolutely, 100-percent a certainty that there is no way that someone could be a little off their game, for lack of a better way to express a potential physical issue, the physical circumstances be compounded by distractions causing someone to misunderstand what they're looking at, and then the physical issues combine with the distracted belief of the surroundings and the person makes an unknowingly risky choice that leads to tragedy?
Seems like if grandpa had a history of being careless with the baby or careless in general at least one of the parents would be out for blood. If grandpa had been an onlooker 10 feet back or simply was in the area longer is there a possibility he may have read the surroundings differently and understood the risk? I wouldn't be surprised if the Prosecution doesn't know the answers to a lot of this, especially if the family departed before extensive interviews could be conducted (not media). If the Prosecution only has immediate, raw reactions from grandpa he understandably may not have been in a state to answer things fully either. Unrelated bystanders were likely traumatized and could have had trouble providing thorough answers and information also. Video footage might reveal some things, potentially not have a vantage point of other things, and even what seems to be very clear, very damning behavior could be less clear viewed in complete context.
The fact time has passed doesn't automatically mean an investigation has led to absolute conclusion, case closed. It may or may not be that the Prosecution hasn't thoroughly investigated the evidence in the light that's not favorable to their conclusion. Good investigation does that. If justice is truly being sought that's a necessary part of an investigation. In order to get a lot of the necessary answers it would be necessary to interview one source for sure, grandpa. That would have been much easier before filing charges and triggering fifth amendment protections. The court of public opinion tends to pay zero attention to the fact that invoking fifth amendment protections can't be used against you, something officials understood very well. Was that a strategic choice? If so, what is the actual aim of the Prosecution? What does justice demand for the loss of this little one? Maybe grandpa was knowingly reckless and should answer for it.
I'm merely providing some counterpoint to the idea that things are cut and dry, black and white as headlines and emotions make it seem. Especially when situations are so tragic it's really important that we get it right, we really work hard for real justice. That's why it's important that we take great care in how we go about pursuing it and don't allow it to be co opted by high emotion OR dollars and cents.