The train from there to Paris St. Lazare in central Paris runs roughly hourly Monday to Friday and slightly less frequently at the weekend. It takes about 1 hour 20-30.
Timetable: http://telechargement.ter-sncf.com/...H_Paris_Caen_hiver_2011_2012_tcm-21-69060.pdf Unfortunately a ticket costs 24€70 (around £20-£21) per person each way (according to www.voyages-sncf.com - English version also available at www.tgv-europe.com).
You then need to get the RER (Réseau Express Régional - Regional Express Network), a surburban line, to Marne la Vallée-Chessy. The RER stops at Auber, a few streets away from Gare St. Lazare. There is an extensive (and in my opinion badly signposted and confusing) network of tunnels linked these two and several other metro stations in the area which you could use. According the website of the Parisian local transport website RATP (http://www.ratp.fr) this journey takes about 45 minutes.
I've done that change at St. Lazare/Auber time or two and it's not that complicated, just quite a bit of walking. I think you can shorten it a bit by going one stop on line 3 to Havre-Caumartin, but I've never tried that.
The main thing is to make sure is that once you're on RER A you don't end up in Boissy-St-Léger.
(Alternatively you could save yourself most of the walk/complicated change by taking RER E to Val de Fontenay and changing there, but I'm not sure if that would save you much time.)
Unfortunately the HN'Go regional railcard is not valid for trips to Paris (this is outside of the Haute-Normandie region). If going to EuroDisney I would buy tickets to Mantes-la-Jolie as long as the train stops there (Tarif Normal is only 16€60 each way) from where the zones 1-5 Mobilis day ticket (14€60) is valid all the way to EuroDisney. This ticket is valid on virtually all public transport within the Ile de France region. I've also noticed 10€ Prem's (i.e. Advance Purchase) tickets on the Bernay - Paris Gare St. Lazare route which you could consider.
If you regularly travel in France do look into national railcards. However if this does not apply (as is likely) I do agree that train travel there is usually expensive unless you are using a Prem's ticket on a long distance route.
Looks like we might be better off with the standard ticket then?
To be fair though, we only have those two trips planed to Paris whilst we're over there. I reckon a rail card might be a little excessive? Everything else we're doing is closer by so we'll be using the car.