If he was negligent, that is to say if he knowingly put his child in danger, then the death of that child should not preclude him from being held to account, and the same would apply if it was a mother.
I think the man who allowed his 7 year old a quadbike was rightly held accountable for his daughter's death for instance.
I think the Mccanns should have been held accountable for their daughter's disappearance, as they knowingly put her in danger...
And if this man knowingly parked a buggy on a slope and deliberately didn't put the break on, then he was negligent and should be held to account.
But if it was a mere mistake, i.e. if he took his hand off the buggy for just a second and it was a tragic accident, then his personal life is no-one's business but his, and his family's.