Travelling with other people means being responsible for documents that are not yours, usually including at least one person who cannot be responsible for their own.
Two categories matter. What has to be checked well in advance, because it cannot be fixed at an airport, and what has to be findable on the day.
Check these months ahead
Every one of these has ended trips at check-in desks.
5 things to do
Ticks are for this visit only. Nothing here is saved anywhere.
Passport validity catches people every year
The rule that surprises people is that many destinations require your passport to remain valid for six months after you arrive or leave. A passport expiring in four months is in date and still refused.
Check every traveller, not just the adults. Children's passports are usually valid for fewer years and expire at unhelpful moments precisely because nobody is watching them.
What to have findable on the day
5 things to do
Ticks are for this visit only. Nothing here is saved anywhere.
A photo of a passport is not a backup
It is useful for filling in forms and for proving to yourself what the number was. It is not a travel document, and it will not get anybody onto a plane.
The genuinely useful record is knowing what exists and where it is right now. Whose passport is in which bag. Whether the insurance is under one person's name. Which parent is carrying which child's documents. That is the information that resolves a problem at a desk, and it is the part nobody writes down.
Keep something on paper
A phone at four percent in a taxi is a normal situation, not a rare one. Passport numbers, the insurance line and the key references on one printed page cost nothing and work when nothing else does.