This was probably the most fun shoot I've ever experienced. That had a lot to do with the kids. They were amazing to work with and made the film set a lot calmer and more entertaining than it can sometimes be. I always wanted them (and their parents) to feel positive and comfortable about this as an experience. Before filming, we took the kids to Paul Hyett's workshop to get them used to the gory stuff. (A warehouse full of exploding heads and severed prosthetic limbs - what's not to like?) After a little coaxing, Raffy, who plays Leah, was pretty happy to stab a fake head in the eye.

After a morning of fun ultra-violence we all went bowling with their screen parents to get them feeling happy with their pretend mums and dads. I realise now that in some ways we were acting out the theme of the film by trying to keep the youngest kids ignorant of the darkest details of the story - I presented everything to them as a fun game. But the kids had a spy - Eva, the oldest, who plays Miranda - who had managed to sneak a read of the script. I don't know why we were worried. The kids were delighted by the idea killing lots of grown-ups. If anything, they were most looking forward to the violent scenes.