It will take nearly 35-40 minutes from Nation Station to Parcs Disneyland (Marne-la-Vallee - Chessy) by RER A. Entrance fee is 40 euro, sometimes there are discounts nearly 50%. One way train ticket is 6.10 euro. If you buy ticket of Disneyland from Metro Station, you will get one way journey free. There are also other places to visit in Disneyland Village. Hope you can be there at least once to have fun.