This is a really hard one isn't it.
For me its full of the head versus the heart stuff!
DD was on a school trip to the house of commons 2 weeks ago and I didn't give a thought to it being risky, even although security at all times is fierce round Westminster. However, if she was going tomorrow I think I would be pretty scared!
I think I would let her go, but slightly easier to say than to do, I grant you! However, there are some important messages that would need to be discussed - i.e. its okay to be scared sometimes but its what we do with that fear that matters, bad things do happen but not all the time, there are evil people in the world but the vast majority are wonderful, lovely souls!
The reason why I think I would let her go, is that if I didn't what kind of a message would that give her of my safety day in day out. Would she start to be fearful for me? I couldn't bear for her as a child to be worrying about me - an adult!
I think you would be very safe if you were just talking delays and disruptions - I'm pretty sure you would find a corner of London to take in a coach load of children. Don't think there is much chance of them being left to wonder the streets