What a good question!
Although I think it is just as irresponsible to have an illegal breed of dog as it is to leave your child unattended, the difference is this:
The grandmother of the child savaged by the dog left the little girl unattended in the same way that she had done probably several times and therefore did it without thinking, and it didn't even occur to her that the child was unsafe.
The McCanns made a premeditated decision to put their children to bed, unsupervised, in a foreign country and leave them for a few hours so they could go out and enjoy themselves, knowing the potential dangers of the situation. I think this is far worse.
I'm not sure the class of the McCanns is what has led to them not yet being ripped apart by the media. I think it is more a case that Maddie is still missing so the focus is on finding her, and judgement will come later.
I don't know why Jacqueline Simpson was held responsible for manslaughter as the dog wasn't even hers.
What she did was thoughtless; what the McCanns did was calculated neglect.