Originally Posted by
shorthauldad
At which point(s) of the journey does that apply?
Counter-example: I've taken several trips without my wife but with one or more of my young kids. Never had even a question about custody or who's who.
Oddly we've (twice) had a minor grilling at the UK Border when the entire family was travelling into LHR all together... (?)
It
can be requested prior to boarding, and it may be requested on arrival (immigration).
Documentation is often not requested in either departure or arrival, but there's always a risk traveling without it.