Oh dear BC. I imagine they were not happy with all the fireworks last night. Poor puddies.
I don't know what type of catflap you've got. We have a very basic catflap which is either fully open or firmly shut with plastic sheet slotted in place inside. They actually broke the flap about 10 days ago and I'm waiting for someone to come a drill new holes to replace outside flap bit. Currently no flap so either a big hole or plastic sheet slotted in place to keep them safe from fireworks.
If we were going away, I would ensure that they couldn't accidentally get locked out again even though that they may mean them having free access but ours normally have 24/7 access in any event.