Hi again
The non-flying bit makes it a bit more difficult. Is she really so set against it? My dad used to really dislike flying but he would do it (he liked his hols!). My DH on the other hand flatly refuses to fly other then when forced to for work. So all our holidays are either by train or car.
If it is by train/ferry you could look at going to Brittany direct from the UK by ferry - to Roscoff or St Malo. I've never been to that part of France but sure someone will be able to tell you more if that appeals.
I wouldn't take the ferry to Calais if you have no car. If you are thinking of northern France I would say Eurostar to Calais Frethun is a better option. There are places that I think are easy to access from there by train, for example Calais to Le Touquet Paris Plage (seaside resort) is only 45 minutes away. Again, I don't know that area very well but to be honest the weather would put me off for a summer holiday there. Also not sure if there is a lot to fill a week or two if you are reliant on public transport.
If you get Eurostar to Paris you would have a lot more options but would still need more than an hour to really get anywhere for the type of place you are looking for. I wouldn't worry to much about a long train journey with the 4.5 year old, I used to do south of Paris to northern england with him on my own from him being about 2 (train to Paris, metro, Eurostar, train in UK), used to take a long time but he loved the trains, meeting people on them, the excitement of the stations etc. Have also done a few other long train journeys with him, and never had any problems at all. Your DS at 8 months may be a bit more difficult of course but still think it is possible. Also depends on how far you have to travel to get the Eurostar as well.
If you think you can handle a longer journey a few places are
Biarritz as Tulpe and DarkSkies have suggested - 5.30 hours from Paris
La Rochelle - lovely old port town, nice restaurants, some beaches, good for boat trips out to little islands off the coast (would love to go back) - 3 hours from Paris
La Baule - seaside town with lovely beaches in western France - 3 hours
Montpellier - old town on the south coast (not sure re beaches) but really nice coast town - 3 hours
Nice - 5.30 hours (the TGV also stops in Toulon, St Raphael, Cannes, Antibes before Nice - so could be a shorter journey if you like any of these
Not a seaside town but Annecy, lovely old town with pretty buildings and canals on the lakeside at Annecy. Very nice place, swimming in the lake, nice lakeside parks and trips into the mountains if you enjoy that (we have spent a few holidays there and it really is nice - Gran would love this one). 3.40 hours from Paris
Just had a thought that as an alternative to going on Eurostar via Paris you could go via Lille. I think in the summer there are services that go direct from Lille to some of these towns. That would mean you wouldn't have to change stations in Paris, making it a lot easier and a bit quicker. Couldn't find any times (timetables may not be out yet for summer) but this site may give you some more information www.raileurope.co.uk/Default.aspx?tabid=39 (sorry can't do the clever one word links
)
Sorry, this has turned into a bit of an epic but hope it has given you some alternatives 