If they don't mind making the trip between Stansted (Stansted Express and Underground) and the London City Airport, than VLM Airlines is also an option, with multiple destinations in Belgium and the Netherlands. http://www.flyvlm.com/emc.asp They will be a bit more expensive, but you have to count the petrol that you would spend if you were to pick them up in e.g. Eindhoven.
The flights you mentioned to Dusseldorf by Ryanair, are NOT going to Dusseldorf, but the small airport in Weeze (nowhere near Dusseldorf) and with pretty bad connections. However there is the option to fly to Eindhoven, take the bus from the airport to the trainstation, and from there into Belgium. (or pick them up there, depending on where you live in Belgium that might be a good option)
And the other option is to take the train. Eurostar non flexible tickets don't need to be very expensive if you book well in advance, and they can take you straight to Belgium. The only hassle you have is to get from Stansted to the Trainstation in the City.
[ Edit: Edited on 25-Jan-2009, at 17:56 by Herr Bert ]