what is the simplest and est way for 2 seniors to get from Paris to Dover, they will need and overnight in Dover to board a cruise ship
Probably the old fashioned way. Train from Paris Nord to Calais (abt 1.5 hrs), then ferry (P&O, another hour and a half) to Dover. It's a good hump from the train station to the ferry port at Calais and I've no idea how the bus/taxi works. Also, my impression is that the Dover ferry and cruise ports are not co-located.
On the Paris-London run there are three Eurostar trains that stop at Ashford International. Those trains depart at 10:13, 15:13, and 19:13. (No 10:13 train on Sundays.) From Ashford it's a short hop on another train to Dover Priory. See the detailed timetables on the German Rail site.
Yes, the most convenient route will be by taking one of the Eurostar journeys that stop at Ashford.