If the pic of the dog carrier next to the pram a few pages back is the actual bag then how the hell was it meant to breath in the first place as I don't see any ventilation holes in it.
A woman with an infant and an 8 year old will come with a heap of hand luggage, nappies, toys etc. so maybe she had already filled the spaces under her and the child's seat and had tried to get away with the dog being in the aisle.
I feel sorry for both the family and the FA but I think the real blame lies with the fact that you can have an animal in the cabin in the first place. Imagine the drama at the escape slide with the pax lining up with dogs, cats, pigs and peacocks (WTF is with that anyway?). I would be pretty p1ssed if I died because of an animal slowing the process down.