For a lot of autistic children, the emergency itself is manageable. What is not manageable is a siren with no warning, a shelter full of strangers, and every routine gone at once.
A simple sequence of pictures — alarm, coat, car, new place — rehearsed calmly on an ordinary day does more in the actual moment than any explanation could, because a child in a heightened state processes pictures and sequence far better than new spoken instructions.
Practise it the way you would a fire drill: not often, but for real, so the sequence is familiar rather than novel when it matters.
Noise-reducing headphones, one specific comfort item, a preferred snack that travels well. For a child who relies on them daily these are not extras — they are the difference between a hard hour and a genuine crisis, and they are easy to forget in the actual rush of leaving.
A one-line card — this child is autistic, may not answer verbally under stress, noise-reducing headphones help — handed to a shelter worker, teacher or first responder does more in ten seconds than an explanation would in the moment. Write it once, keep it with the go bag, and it is done.
Warn them. The goal of practising is familiarity, and a surprise defeats that goal for a child who relies on predictability specifically.
That is worth working out in advance with your local emergency management office, not in the moment - ask what accommodations exist before you need them.
Part of our guide to Talking to kids about emergencies.