Train travel

The sites you need are http://www.firstgreatwestern.co.uk/ for Marlow-London (only for timetable check, there's no discount for buying tickets in advance as it's not far outside London) and www.eurostar.co.uk for London-Paris. There may possibly be some cheap fares available for London - Paris (although you've left it a bit late) but the cheap fares tie you to a specific train that you must not miss. The train from Marlow arrives at Paddington station in central London: you need to get to St Pancras for the Eurostar to Paris. The quickest way is by taking a Circle or a District Line Underground train one stop eastbound to Edgware Road and then a Circle or Hammersmith & City train further eastbound to Kings Cross St Pancras station. You must check in at least half an hour before the Eurostar departs at St Pancras. I would add at least one hour and preferably ninety minutes on top of that as your target time to arrive at Paddington if you have no experience with trains or subways and are travelling with children. A taxi would be easier to use and possibly not much more expensive for four, but MUCH slower if you are travelling in the evening during the commuter period. The train from Marlow to Paddington should not be too busy as most people will be heading in the opposite direction in the evening, but the Underground will probably be very busy.

An additional thing: have you already booked your hotel in Paris, and do they know what time you plan to arrive. Many smaller hotels in Europe do not have a 24-hour staffed reception, so if you are arriving in the evening, you should really contact the hotel and make sure there will be a person there to let you in. Remember also that although the Eurostar travel time is 2 hours, France is in a time zone one hour later than the UK, so the effective arrival time in Paris may be later than you expect.