Hi we will be traveling on the 7th September with 3 kids and we want to go from Disneyland to Gare Montparnasse. Which is the cheapest and quickest way ? ReR A to Nation and than metro from Nation to Montparnasse or ReR to Chatelet and than metro to Gare Montparnasse ?? Also what’s the timetables ??

RER A from Marne La Vallee to Nation station, transfer to Metro line 6 direction Charles de Gaulle-Etoile until Montparnasse Bienvenue station.  Walk to Gare Montparnasse from there.
 
Trains are frequent.  Every 10 minutes roughly.
 
Buying single use RER tickets at Marne La Vallee will be cheapest option. 7.60€ for each adult and 3.80€ for any child 9 and under.