Train to Brussels

Yes, but the trains will be less frequent than ones from central Paris. Check times on http://www.bahn.de, picking trains from "Aeroport Charles de Gaulle - TGV" to "Bruxelles Midi". I get trains on Sunday at 1013 and 1235. (Only trains which show TGV with no other letters under "Produkte" are through, the others are going into Paris by RER and getting a train from Paris Nord).

You just walk to the RER station in CDG, go down stairs, and buy your ticket. Can't tell you if their are fewer trains on Sunday or not, however, we had didn't have long to wait after we bought our tickets.
Note that your ticket will give you your car and seat number along with the track. Their will be a picture or the train near the center of the platform along with a guide of where you should wait on the platform to board--it doesn't spend much time stopped at the station so you'll want to be as close to your car with your baggage as possible.

You also can take a TGV from CDG to Lille and connect there to either another TGV or the Eurostar to Brussels.

What's your travel date?

Is Bruxelles Midi the same as Brussels Centraal?

No, Brussel-Zuid is the same as Bruxelles-Midi. Going north through town the next big one is Central, then Nord.

There are three main railway stations in Brussels - Midi, Centrale and Nord. TGV, Thalys, ICE and Eurostar high-speed trains only stop at Midi, but non-high-speed trains stop at all three. If you have an international ticket into Brussels by Eurostar or TGV, you should have a free connection to any Belgian station, so if Nord or Centrale is more convenient to where you are staying you can just get any Belgian domestic train heading north through the city.

Thanks for your help everyone. I checked the TGV web site and we can get a train at 7.47am and 9.47am. Despite going for a month only have carry on luggage so hope to clear the airport in reasonable time.

Tim our flight ands at 6.05am on Sunday 20th June.