You do not take another person's five year old anywhere other than where you have been given permission to go. If this means picking up your own kids at school late then so be it. You just simply do not do it.
Depending on the ages of the other mom's kids, their *actual* safety (If they would have been old enough to be left standing outside, or left to walk home on their own) and their *actual* emotional comfort (if they were worried about where their mom was--I remember imagining that my mother had decided to abandon me) might have been genuinely at risk.
The OP's child was completely safe. He was probably also completely happy and entertained, etc. He was never at actual risk.
I sympathize with the OP--truly, I do. I understand why she was upset.
I don't think this mother should have volunteered to drive if she had any afternoon deadline (which she did).
I think the preschool needs to pay closer attention to the situations of those parents who become "authorized drivers" for trips, and reject any who have other commitments like picking up their own kids from school. They need to *not* say, "Oh, we should be back by that time."
I think the preschool needs to create better webs of communication. What if this woman *had* run into automobile difficulty--a fender bender, a flat tire, etc.? She needed numbers of several people that she could call.
I think the OP is perfectly within all bounds of etiquette to pursue these points with the preschool, using this unhappy experience as her leverage.
I hope she does, because this was not an ideal situation at all.
As for her original question, what to say to the mom: The OP has already glared at her, so the mom knows the OP is unhappy about the situation and blames her for [as the other mom would see it] "not sacrificing her own children for the OP's son." What else is there that the OP wants to communicate?