KatyMac
Amanda's husband here, I know a bit about French trains (don't ask!)- with or without kids!
Getting to Bordeaux from Britain by train is quite easy, though a lot depends on where in this country you are going from, number and age of kids, number of adults travelling and amount of luggage etc...
As others have said its Eurostar/TGV (French High Speed Trains) to Bordeaux. You've basically got two choices:
1 - Eurostar London (or Ashford/Ebbsfleet) to Paris; then across Paris by Taxi from Gare Du Nord to Gare Montparnasse and TGV to Bordeaux.
2 - Eurostar London (or Ashford/Ebbsfleet) to Lille Europe; then a TGV from the same station (sometimes even the same platform! to Bordeaux.
The 2nd is much easier; the only drawback being that there are far fewer trains this way (2-4 a day). This means you have to work around the train times, it also means you need to be organised with booking tickets as the cheap fares will sell out more quickly.
Note that the tickets are only on sale 3 months (I think, it may be 4 on Eurostar now) in advance - you need to be organised, find out when they are on sale and BOOK THEM as soon as they become available. Unlike airline tickets, Eurostar and SNCF tickets simply start out cheap and get more expensive in my experience - book as early as possible.
Typical journey (this years times they'll change for next summer but probably not much; you'll find out when booking) would be London 08:39 Lille a 11:26 d 12:16 Bdx 18:00
If you're inexperienced in Euopean Rail travel then maybe use an Agency rather than buying direct from SNCF; www.Raileurope.co.uk is one I often use. But make sure they offer you the really cheap SNCF tickets called 'Prems'. Also look at the excellent website www.seat61.com
Bordeaux to the site is a bit different.
Ychoux is indeed the nearest station, but in common with many small town station in France the service is sporadic to say the least, with only a few trains a day. This might mean a very long wait at Bordeaux, or problems if your TGV was late. You'd also want to ensure that any taxi was prebooked, as there may not be much happening at Ycoux station! Example timetable here>
www.ter-sncf.com/Images/Bordeaux%20-%20Mt%20de%20Marsan_tcm17-21384.pdf
and map here>
www.ter-sncf.com/Images/11-Mt-de-Marsan-Bx72dpi_tcm17-6243.jpg
You might be better off hiring a car in Bordeaux, though that would mean negotiating an unfamilair city centre - depends how confident you or whoever is travelling wit you is!
A 3rd alternative would be to instead catch a train to Arcachon, still 20 miles from the site but a much bigger town with a better service from Bordeaux.
www.ter-sncf.com/Images/Arcachon%20-%20Bordeaux_tcm17-21372.pdf
You could probably hire a car here (though you'd need a check) and get a taxi (though it would be dearer than Ychoux). And of course it's a 3rd train.
Ferry to Bilbao I'd probably not do; Bilbao to SW france by Train is too infrequent/hassly in my view. (though I can recommend Northern Spain, either via Bilbao/Santander as a great holiday as long as you don't mind not having wall to wall Sun...)
This has probably made it all sound more complex than it should; I hope not- with a little knowledge and a bit of planning train travel to many parts of Europe can be a wonderful way to go on holiday - Cost Effective, Envioronmentally Friendly and fantastic fun for kids - espeicially compared to anxious delays at Airport security or endless hours stuffed in a car.
Once gain please go to www.seat61.com - hope it might convert you and anyone else reading this!!
(and no I don't work for the website or the railways, just experienced in European Travel and believe that there are alternatives sometimes worth considering.....!!)
Hope this helps and Good luck!
Antony