One bag per person, packed now, kept where you would grab it. Packing it later is the plan everybody has and nobody executes.
In it. Copies of documents. Water for a day. Food that needs no preparation. A torch and batteries. A power bank and cable. A small first-aid kit. Medicines. A warm layer and waterproof. A change of socks and underwear. Soap, toothbrush, sanitary items. Cash in small notes. The written contacts. A whistle.
Weight is the whole design. You must be able to carry it, while carrying a child if you have one, for as long as it takes. A bag too heavy to carry gets abandoned with everything in it.
One per person, including children, who can carry their own small one — and should, because a bag of their own is one thing they control on a day when they control nothing.
Check it twice a year. Children outgrow clothes, food expires, batteries die and documents change.