As others have said, take the RER B towards Aéroport Charles de Gaulle/Mitry-Claye and get off at Chatelet-Les Halles (it's 4 stops from Denfert Rochereau). Then, get on the RER A towards Marne la Vallee/Chessy/Parc Disneyland. When you get off the RER B, you'll have to walk upstairs and cross over to another platform. There are tons of signs pointing you in the right direction!
I lived at Cité Universitaire, which is one stop south of Denfert Rochereau. When I timed it, it took me exactly 1 hour from leaving my apartment to get to Disneyland. But that obviously can vary depending on how long you have to wait for the train. If I were you I'd leave at 8:30am!
Just a tip, make sure you check the signs above the platforms before you get on a train to make sure it is stopping at the right stop! Most of the B trains stop at Chatelet since it's a large station, but you might have some trouble with the RER A. That line splits, so some trains don't go to Disneyland, and even some of the ones that head toward Disneyland don't go all the way to the end of the line. Just check the signs and you should be fine!
EDIT: All of the B trains stop at Chatelet, and there actually isn't a sign naming all of the stations at Denfert-Rochereau. But any train you get on headed north (Charles de Gaulle or Mitry-Claye) will stop at Chatelet.
