I had something very similar happen several years ago. Dcg (4 yo), took off leaving me to drag 4 other kids after her. Very long story short, it culminated in me having to run in front of a Mac truck to grab my 2 yo, who was wandering right down the middle of a busy road, while the dcg played in a puddle on the shoulder of the road! I still get so angry when I think about it. Dcg sat in timeout for the rest of the day, and my son (who took off after her) got a spanking and spent the rest of the day in his room. I was seething! And dc mom laughed it off!
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Anyway, I know exactly how you feel, and all I can say is make sure your dc mom knows exactly how dangerous this behaviour is. I'd also probably make them find another way to get back from preschool, like a pp said. But whatever happens, I would definitely NOT trust this dcg again!