An evacuation backpack for a child is a kit of their own that your child can carry themselves. That is no detail: if you have to leave the house, you need your hands, and a child who carries their own bag is calmer than a child who is just dragged along.
What belongs in it?
Water and a ration for at least 24 hours, a flashlight, an emergency blanket, a rain poncho, extra clothing, a card with names and phone numbers of parents, and something of their own: a cuddly toy, a book or a game. That last item seems superfluous and it is not.
What weight can a child carry?
Use about ten percent of body weight as an upper limit. For a child of 25 kilos that is roughly 2.5 kilos. Divide the heaviest part, water, over the parents' backpacks.
Talk it through once
Let your child carry the backpack themselves once and explain when it goes with you. The government's new Denk vooruit campaign explicitly asks for this: make arrangements and talk about them, not just bring supplies into the house.
Keep exploring
For the whole household there are emergency kits for families. Separate backpacks and waterproof boxes are under backpacks and storage.