You can book your ferry crossing using Airmiles/Tesco vouchers if you have enough through Airmiles - not sure how much it willbe but with pre schoolers def go out of season!! We have done 5 years in June or Sept which is much nicer , quieter and better weather than Aug - both Brittany and Vendee!! Also prices are MUCH cheaper. We stayed at Le Guilvenic Village la plage which was a lovely location but site is very basic and toddlers pool isn't really suitable for toddlers! Ds was 3 and dd 2 and the pool was crap for them/ Small very deep and with very high sides. as I said location great as ona beach but pool - no.
best pool complexes for kids your LOs age that we've stayed at are;
Raguienes (brittany) - camping les deux fonataines - FAB pool for LOs and adults. Mini disco early eve, walking distance to beach. Driving distance to UK run indoor soft play - in case of rain! has baby area.
Camping Acalpulco in St jean de Monts - lovely pool - sandy area play area, great location - near beach and local shops/suopermarket. Great restaurant/takeaway. would go back
camping Mane Guerneheu, baden Brittany. Lovely lovely site with great pool complex, great restuarant (with little tikes play area - ideal for an 18mo dd and 30 mo ds!!!). 3 fishing lakes - feeding ducks and 2 fields of donkeys and goats we could feed! As we went end Aug/beg Sept they were closing down kids club so our kids were allowed to play in the kc tents and use up all the stuff!! Also within wlaking distance of a lovely village with all you need. had to drive to beach. Only downside is that siteis hilly. means if you are bottom of hill, you ahve loveley views n peace and quiet but hilly walk to pool!Didn't bother me but it did Dh!
Also been to Camping les Bois Masson in St Jean de Monts but wouldn't go back - on main rd, haven Europe sitea nd v British n busy!
If you sail to ST malo I'd recommend the overnight sailing - dead easy with LOs!! Tire em out at the live band in the bar while you ahve a vino or 3, then bed in the cabin. Wake up and you've a couple of hours drive to resort! fantastic and offordable out of season. Sadly, ridiculous prices out of season! Good luck choosing!!
You can pay for holiday with Twesco vouchers if you go with French/Canvas Life or Siblu.