KM
Sorry I missed Domain du Dugny!
Onzain is fairly well served from Paris (every hour or two , takes 2 hours)
Prices vary, as they do here on trains (and indeed on Airlines). La Rochelle, for example, can be done for as little as £400 from London and back all in for 2 Adults and 2 Children(via Paris, no Lille options for La R I'm afraid) but these fares need to be dug out, and may be more difficult / impossible on say Saturdays on Summer. Did I mention BOOK EARLY?? and www.seat61.com Fares then rise in stages up to £1000 or more for the 4 of you for that trip. Onzain is more like £300 (via Paris and book in 2 stages) with the Paris to Onzain bit being easy to get cheaper fares as it's not on TGVs, it's the Eurostar bit that you need to book early here for, again can rise to a lot more than that.
Eurstar and SNCF TGV pricing similar to airlines in many ways now (although no 'last minute' bargains and the train pricing is far more transparent than budget airlines).
Main advantages of train vs flying are the much better general experience (almost part of the holiday!) especially with kids, the transparent pricing with no little 'add-ons', the lack of baggage restrictions and security hassles and of course the environmental angle.
Main disadvantages are length of time it takes to anywhere other than Paris/Lille and from anywhere outside SE England (it's a whole day travelling really) changing 1 or 2 times (especially if you need to cross Paris with luggage AND kids*) and effort and pre planning required to get the cheap fares vs airline websites who make it easy to book flights. (If the railways got thier act together, increased options from Lille and got a decent website and pricing structure they'd clean up. Maybe one day....)
- The transfer from Nord (Eurostars) to and from Montparnasse (TGVs to Brittany & South West France including La Rochelle ) by metro Line 4 is particularly frustrating. After 14 stops on the often crowded metro, you arrive at the ironically named Montparnasse Bienvenue mtero station. The 'welcome' in this case being the fact that the metro station itself is bloody miles from the mainline station, with various corridors, travelators and escalators to negotiate. Ok if you're on your own, but with holiday luggage, tired kids and tight connections it isn't fun! (take a taxi, or better still, ensure you leave plenty of time). Nord to Gare de Lyon (TGVs to South of France, Provence/Languedoc etc.) by contrast is easy, 2 stops on RER line D and much less walking. Nord to Austerlitz (for Onzain) not too bad either, line 5 with not too much walking.
Anyway, back to the train vs plane to France debate....
Problems, and in particular level of contingency if things go wrong is an interesting topic. Trains aren't perfect, as anyone who caught Eurostar just before Xmas will testify, strikes do happen and SNCF customer care is as variable as you would expect from a French state owned organisation, ranging from quite good to the verbal equivalent of a Gallic shrug of the shoulders. On the other hand the reliability and punctuality of the trains is, I reckon, better than short haul flights and the amount of people posting on the Internet with horror stories about budget airlines (one in particular!) and how they treat their 'customers' when things go wrong tells it's own story I would say...
We've done all sorts over the years, trains, planes, motorail (when it still existed) and driving. They've all got pros and cons, but all else being equal I'd take the train every time.
Good luck anyway!