In both London and Paris, I would venture to say that they could/will take the train everyday to go see the sights, it will just be a Tube/Metro train. With the quality of public transit i.e. number of stations, there is no reason to pick a hotel near the long distance train station. Either cab/public transit to/from it.

<<have them take the Chunnel train>> No-one in the UK ever refers to Eurostar as the Chunnel train, assuming that is what is meant