DH and I are married, have the same surname and have a DD(16). Occasionally over the years we have gone away separately with her. I have always scoffed at the slightly alarmist MN threads about being stopped at airport security and being asked about permission to travel but often prepped an appropriate letter/ documents JIC.
Completely forgot this time (and DD is 16 so practically an adult) and for the first time DH & DD have been stopped at a border. Just got a panicked phone call from South America. I've had to send through a letter, her birth certificate and my passport via WhatsApp. Waiting to hear back if it's okay...