Your daughter should have communicated with you better before they went away about what level of freedom your granddaughter was normally allowed. Then you could have decided to what extent you were to be used as a taxi for her late returns and to what extent she needed to make alternative arrangements for getting home. You could point out to your daughter that on occasions when she wasn't available to ask, you felt sure she would have been even MORE annoyed if you had made a decision that left the young lady exposed to more danger than her own mother would have allowed. It's completely correct to default to the safer, more controlled option, when in doubt.
Lack of public toilets in towns.