I am shocked that they would leave without her. I can't believe they wouldn't give a call to any of the girls who were not there to make sure they weren't lost, waiting at the wrong spot, etc. B/c in girl scouts, they would have had to fill out a permission slip with emergency contact info and probably pay at least a small fee. so I would be concerned if they didn't show up then (as a leader).
It might just be our troop, but they really emphasize showing concern for others and that would include checking on someone if they weren't where you expected to make sure everything is OK.
And I think the pile on to the OP is uncalled for. The departure time was 6:15 - unless they were instructed to be there early, that doesn't mean it's wrong not to arrive early. Actually, as an organizer of an event, you should allow some cushion for others clocks to be a little off from yours. These GS leader just up and broke a little girls' heart over what couldn't be more than five minutes. It's not like the OP said that they arrrived at 6:32 or soemthing and had expected them to wait. And now they are not calling the mom back? What mean women.