Disneyland for the day from Charles De Gaulle airport

A word of warning re transport - the only direct trains between CDG and Marne-la-Vallee (for Disney) are very expensive TGV trains (walk-up fare is over 20 euros, you might get a cheaper fare in advance but that will limit you to a specific train). If you use local trains, you will have to travel all the way into central Paris on RER line B and out again on Line A, which will take a long time and probably be very crowded at the times you're travelling. There is also a bus route at http://magicalshuttle.fr/#, but this is also 20 euros each way.

Take the rer b line from CDG to chatelet les Halle's metro stop about 40 minutes , then the rer a line to Disneyland , about another 40 minute ride .chrck out the Paris metro maps on line to get an idea how the metro and trains work ,

http://www.rome2rio.com/s/Paris-CDG-Airport-CDG/Disneyland-Resort-Paris

gives the overview, although for Dec 26 the TGV tickets are closer to 30 euros.

Maybe go all the way to Disneyland from Belgium, stay there, then get a taxi to the airport early in the morning. Or spend 1 night in Disney and the last night near CDG, or rent a car for the day from CDG.

Since it costs 10 euros just to get into the city on a slow crappy RER train, I think getting out to Disney direct for 20 euros on the comfy fast TGV would be pretty darned good value in this instance. Why you'd want to RERB into the city and then RERA out to Disney if you have the TGV option -- or, even worse, rent a car and drive -- is beyond me.

Money is not the only finite resource you have when you're traveling. Two others, which are relevant in this case, are time and your sanity/patience. It is not worth saving a few bucks and giving up time and ease in this instance.

Kim is absolutely correct about avoiding going into town and then back out to Disney on the RER A, as well as avoiding renting a car. I'll also add, taking a taxi for such a long distance. There are several van services if you get stuck.

Rome2rio.com shows a train from Brussels directly to Disneyland Paris every two hours, which takes 1:48. How does that compare to Brussels-->CDG-->Disney?
You could go Brussels-->Disney, stay the night and then go on to CDG for your last night?

As I said, TGV fares around Christmas are running 30 euros so 30 x 4 people then doubled for the return trip is a pile of money to spend going such a short distance (240 euros). Renting a car would be half that and liberating schedule-wise, no rush to meet a set train time to return. The best option though is my first, to stay near Disney and get an early taxi or shuttle to the airport the last day.