YABU - I understand your thinking, but I think you've gone a bit wrong here
I think you had three options
- You swap with another adult so the trip continues as planned without you
- All the other parents come and collect their kids and the trip ends
- You sort transporting the girls back home and the trip ends.
I can see why #1 is the preferred option, but the first port of call should have been calls to the other parents to say "I'm going to have to leave asap due to an emergency, is anyone able to come and supervise the trip?" If the answer was no, then you could have said "Right, Sally, my older daughter and her boyfriend are available and willing to come and supervise. Is that ok with everyone? If not, the trip is over"
If all said no then that should have been a "right, well trips over, is anyone able to come and collect the girls so I can go straight to deal with my emergency?"
If not, that should have been a "right, well the girls will be back at the location we agreed I'd drop them off at in about 5 hours time then, please be there too". If only one parent said no, then you'd probably be wanting to say, "right, well the options are you collect her asap, or I'll drop her off in about five hours"
If everyone had been unhappy, you'd have genuinely needed to leave in the opposite direct asap, then perhaps a compromise could have been you bought all the girls train tickets on a direct train, took them to the station, saw them onto the train, and another parent or everyone's parents collected them at the other main station, so no changing trains needed. But again, you'd need parents and girls to be happy with that.