Just a thought, in terms of the storyline, and plot needing two children to be able to hide on an 'inner circle' train of the right era...
Without knowing the detail of the rolling stock in use at the time, but just wondering would it have been feasible for them to hide in a driving cab or guard's compartment in the middle of the train? By the time I remember the Underground, driving cabs in the middle of the train (where the two units joined) needed either a key or a 'break glass' type handle to get into, but would that have been the case in 1941, or would there just have been a polite notice saying no entry, or words to that effect? The Q stock driving car at Covent Garden (flickr photo
here - not mine) appears to have a door-knob rather than something needing a key.
And again, on the underground stock I remember (I have a very faint memory of the CO/CP / R stock generation)
the guard positions were effectively part of the saloon, but think I've encountered one earlier carriage (in the LT Museum collection?) where the guard position had a hinged door, and seemed more closed off from the rest of the carriage. Would this - along with the effect of wartime / blackout reduced carriage lighting - have offered a hiding place?