Unfortunately some people just aren't great on either communication skills or manners. Drives me bonkers as in my mind it is just automatic. If I were in her shoes I wouldn't have thought twice to give a quick courtesy call to either ask if you still needed me or to inform you that I wouldn't be walking them as it isn't safe, for the same reasons playfelt said. If I was in your shoes and she wasn't here at her regular time, I would have actually called after about 5 minutes to ask her of her whereabouts. That way, she would have still had the opportunity to come around to your place and walk the kids to school and any initial assumptions made about this would have been set right immediately and the whole situation rectified within minutes.