Op, break it into steps
They'll be herded onto the train and then stay there
Then train to tube. Once past the escalators, it's very, very easy to control a group in the passages as they're mostly white or beige tiled and very well lit and there's not actually that many places to wander off too at most stations
Then a closed train again.
The science museum is reached by a tunnel directly from the tube station - no roads.
Up to the museum across the gardens. Into the museum and navigated in small groups.
Back to the tube, to the theatre, same as before.
At that age, most of the kids will be fascinated by the maps and the lines and how it works
Theatre's safe - again a closed space with lots of staff.
Back on the tube.
Back on the train.
The only things I'd check is -
Can your child recite their name, address and preferably school name and a phone number or can you attach it to them somehow
Are they okay with escalators? There's a lit of them and they move pretty quickly.