Well I still think that I've already given you the easiest route.
To use a train, you'd first have to to take the Tan Air Shuttle bus for the 20 minute journey to Nantes rail station. (Fare €7 per person each way. Buses every ½ hour Monday to Saturday, hourly on Sundays).
Then you'd need to take a train to Challans. That's a 1 hour journey (with a rather irregular service - you might need to wait for over an hour for a train)
From Challans you'd need to take the connecting SNCF bus into St Jean De Monts. That adds another hour hour, or more (depending upon the connection), onto your journey. The inclusive adult train & bus ticket, from Nantes rail station to St Jean de Monts costs €13.20.
So each adult would be paying (2 x €7) + €13.20 = €27.20. But you'd probably still need a taxi to get to your hotel. I fail to see how that could be cheaper than my original suggestion. (You might save a very small amount by taking a taxi from the airport to Nantes station, at around €25, instead of paying for separate bus tickets, but it wouldn't help much).
I can't find any suitable bus-only routes for your journey.
Chris