Lost my wifi at home so thanks all for your replies, back on now!
I have read all of your messages, it made me reflect why I was worried.
To summarise - I am a Londoner, have been communting into town since I was 18, love London. But the tube/trains are a different story for me in terms of getting caught up in or outside the doors, crowds, dodgy people (and there are loads), I was caught up in the 7/7 bombings at Kings Cross also and it's never been the same since for me, but I still commute as I would not allow it to stop me. And yes, the 'impending' threat to London is certainly there, my workplace is made aware of any threats as and when they happen as they need to be high security, so i am aware of some of these details too.
My other worry is my 5 yr old boy is more than capable of trying to show off to his classmates by doing something stupid like going near the platform edge when asked not to. But he does also listen very well to his teachers so he may behave ok.
Parents can only be helpers to children in a different class - not their own.
Both me and OH are very easy going and let our children experience whatever they can so that's why this one is bothering me. Not allowing him to go goes against my principles, I think my worries have got the best of me. Both our boys are always up for an adventure.
We have taken them into London many times, they have absolutely no sense of danger at this age (5 and 3) but they have experience of the tube.
BTW the shop is Hamleys to the person who mentioned earlier - i do see it as a bit pointless but guess they are studying toys this term. And yes I know DS1 will love this trip.
Thanks for making me sit down and rationalise it inside me head, I will let him go although I will still worry, but if i dont it will only make me regret it for a very long time when most of his classmates are talking about it, plus it's a good experience.
7 parents have opted not to allow theirs to go so far, and it's actually all the parents who commute into London everyday.