If she genuinely did left the child intentionally in the car to go off for an hour, then I hope she is penalised in some way and I hope, under the circumstances her car insurance doesn’t pay out.
However, the sticker on the car doesn’t prove how long the child was alone. Maybe she put the child in the car as he was sleeping, took the oldest to the toilet and didn’t realise how long she’d be, as there was a fair on. I’m not saying she was right and I wouldn’t have don’t that myself, but she shouldn’t be demonised before the fact are known.