Everything after the first day runs on being able to prove who you are. Crossing a checkpoint, claiming assistance, registering at a shelter, being reunited with a child, making an insurance claim — each of them asks for a document.
What to have with you. Identity documents for every person travelling, including children. Anything that proves a relationship: birth certificates, custody papers. Medical documents for anyone with a condition that matters.
What to have somewhere else. Photographs of all of the above, stored where you can reach them without your own phone — an email to yourself, a cloud account whose password you know without looking. A copy you cannot reach is not a copy.
Cash. Card payment depends on electricity and a network, and both fail together. Several European civil-protection agencies now say this explicitly in their household advice: keep some cash, in small denominations, because nobody has change.
What not to carry. Anything you cannot afford to lose and do not need. A bag heavy enough to slow you down is a bag that gets put down.