For any good parent or grandparent that would be their worst nightmare - and for Mr Hartley that is what it was.
Describing the impact on his life, he said he could not speak for two days after the attack and did not leave Tainui's bedside for three weeks.
Judge Stephen Coyle spoke of a grandfather's close relationship with his grandson in which Mr Hartley was the most important male figure in the boy's life.
In this day and age so many kids lack a good male role model.
He was spending time with his grandson when the attack took place and cannot be blamed for it.
He was a responsible dog owner and it is obvious how much he cares for his grandson.
If he could have stopped it from happening, I have no doubt he would have.
Judge Coyle said Mr Hartley's remorse and trauma was far greater than any punishment the court could impose and Tainui would be a tragic reminder of what happened. I agree. Mr Hartley has suffered enough. There was no need for any further punishment.
I understand the need for the case to be brought before the court. We have laws for a reason and people cannot simply be pardoned without going through the proper process - even if it is a difficult and painful time for those who are eventually found to be innocent or without fault.
Mr Hartley's case is exactly why we have a judicial process and shows the important role of judges.
If we simply relied on the letter of the law he would have been convicted but just because he owned a dog who attacked someone does not mean he is guilty of any wrongdoing.
It's the humanity and compassion of judges (and juries) which makes our legal system work. Judge Coyle could see that Mr Hartley meant no harm and had already suffered enough.
On the flip side of that is the case of baby Moko's death.
The toddler was beaten to the point where he suffered facial swelling, internal bleeding, septic shock from his leaking bowel and swelling of the brain and was left in that state for four days before his caregivers called 111.
He was kicked, slapped, stomped on and bitten by those entrusted to care for him.
On the first day of their murder trial Tania Shailer and David William Haerewa pleaded guilty to manslaughter.
Unlike Mr Hartley, Shailer and Haerewa obviously did not care for the young boy in their charge.
I have no doubt the judge will have some harsh words for the pair when they are sentenced but in this case the outcome was decided before the point in the judicial process where the humanity and compassion of a judge or jury could come into play.
The case prompted Canterbury University law professor Chris Gallavin to describe New Zealand's homicide laws as a "complete mess" and Sensible Sentencing Trust also slammed the Crown's decision to not pursue murder charges.
In this instance, I am inclined to agree.
The case of baby Moko highlights one of the major failings of our justice system. I understand that by giving the pair the option of pleading guilty to the lesser charge the Crown was guaranteed a detailed confession and a conviction but at the same time in my view it doesn't seem right.
Mr Gallavin is calling on the Law Commission to consider introducing different degrees of murder into our justice system for cases such as this.
That seems like a sensible solution.
Even so there will always be cases where it seems as though justice has not been served.
I'd like to hope that each of those cases sparks consideration of what can be done to improve the system.
But, we must also remember the good our courts do and the care and consideration our judges put into making decisions which will change the course of a person's life forever.
Whether it's sentencing an offender to prison and rehabilitation to punish them for a crime they are found guilty of or discharging a devastated grandfather without conviction, our courts and our judges do a lot of good.
Theirs is a job I don't envy.
