Talk about One Strike and You're Out! They do sound like they are overreacting - particularly the "15 years as a Head" b?ll?cks - for goodness's sake, if it's the first time this has happened she's led a very sheltered head-teaching career!
But they do have to cover themselves, which I guess is why all the palaver - just seems rather OTT to me. Ds1 did a similar thing when he was in Reception - though not on a school trip. He disappeared at playtime, they didn't realise 'til they had all been back in the classroom for quite a while. He was in a different part of the playground (where he shouldn't have been) and hadn't heard the whistle - they sent out a 'search party' to retrieve him. They talked to him about it, told me, and asked me to talk about it to him too - the importance of not wandering off. That was deemed sufficient and he never did it again. It seems to me that thay have labelled your child already as a serial absconder; probably because the "what if" scenario of her wandering off frightened them to death.
Also, I know they can't watch them all the time - but on a school trip they are particularly conscientious - should have a good ratio of adults to children for 5 year olds and should be doing constant head counts - so possibly they are feeling liable in this and seeking to turn the blame onto you?
I would go in again, say you have had time to think about it, you have talked to your dd and are fairly confident she will not do this again. But I would be adamant that if my child had to wear a wrist band then so should all the other children - it would be in their interest anyway.
Bit of a waffle, but HTH!