Should parents be held responsible?

I know accidents happen. But a lot of accidents could be avoided if people payed more attention to their kids. 
I know a couple that had a 2 year old and a 4 year old. One day when they were sleeping all day the 2 year old got out of the house and drown in the next door neighbors pool and wasn't found until an hour later. I know that's devastating and horrible but do you think they should have been held responsible because they were asleep all day and just let their kids do whatever they wanted without supervision?