This is certainly tragic, but I don't know if I can say I feel sorry for the mother. Heck, there are times when my 4 y/o is not with me and I still start to open the back door to get him out. I understand accidents happen, but we're talking an entire 8 hours she left him in the car without realizing!
As far as the daycare calling, my son's daycare doesn't. It's their policy that the parent is supposed to call in, but there have been times I haven't and they don't call. It's a daycare with young children. I'm sure there are many kids who don't go every day. My two older kids school calls me if I don't, but I think daycares are used to kids missing days here and there. This should not be put on them.
The way I look at this accident, parents are supposed to protect their children. If they don't, they should be held responsible. If I accidentally left a gun out and one of my kids shot the other, would you say "oops, it was an accident". If I forgot to buckle my kids seat belt and they died in a car crash, would it be "oops, it was an accident"?
There's a big difference between a tragic accident and child endangerment.