No excuses. Charge her with something.
I have real trouble believing a child can be forgotten in a car. Everyone I see getting out a car turns back and glances at it. Women have purses they turn and pick up and look in the back seat.
Frankly I think this is just plain murder and excused away as forgetting. I notice too this happens in the summer so often and not during the year when the tempatures are mild.
Most if not all cars do not allow you to see directly behind you. Meaning you can't turn your head 180 degrees and see the back seat. In a 1965 Chevy you could see anywhere in the car real easy. Your own seat was not blocking your view. You may glance at the car as you leave but what do you see?
Cars are much smaller and difficult to see both into and also what's going on inside the car while your in it. Car seats are low profile, often hidden from review mirrors, add tented windows and you likely see nothing but maybe the lock knob as you leave.
Personally I loathe the car seats. A kid that is a year old and definitely a kid 18 months old if not strapped into a car seat like a space shuttle launch can most likely get out of the darn car themselves if they could get out of the darn car seat. The left in car incidents most likely increased with the car seat law. Not even a five or six year old can get out of some of those things alone.