OP, it is perfectly possible to do a weekend getaway to DLP from Waterloo. Heck, it's exactly the same as me going a weekend, since I'm from Brussels.
By car, it takes about 3.5-4 hours, depending if you do some stops. We usually stop at the first parking in France to eat our breakfast (that we bring from home) and between peage at Senlis and CDG to use the bathroom
From where I live, it's about 330 kms to DLP, from Waterloo it shouldn't be much further.
Driving directions:
Take R0 (ring around Brussels) and follow E19 Mons - Paris. The E19 will become the A1 once in France. Péage is 12.40 EUR. At CDG keep right, and follow Marne La Vallée, which will bring you on the Francillienne. At the end of the Francilienne (I think about 20 kms), you will NOT follow Paris, but Reims - Metz-Nancy, whatever, not Paris, and that will bring you on the A4, around exit 9. You'll need exit 13 (I think it's 13, there is a sign that says Disney theme parks or something like it).
The tricky point is at CDG, the signs are not very clear, and the Francillienne is not marked very well. After the tunnels, just keep right and follow Marne-la-Vallée.
When driving back, to the inverse, and make sure you don't forget to take the A1 to Brussels instead of Lille (it splits at a certain point).
There are a couple of fixed-cameras on the A1 for speed controls, so don't drive too fast.
Going by train: look for a way to get to Brussels South station (Brussel Zuid - Bruxelles Midi), train should be the easiest, and then take the TGV to DLP. Easy, quick and relaxing. I always find it horrible to have to drive back home after a DLP-visit. Now that we go often, we leave earlier on departure day, but if you only go once, I would take the train.
Be sure to make train reservations in time, the earlier, the better and the cheaper!
If you need more info, just let me know.
