Hi -
I'm assuming that as they're only there for one day they will be willing to go for it. I tend to disagree with some other posters in that the beauty of DLRP is that as the two parks are next to each other you can see both in a day.
First off I'd check opening hours here:
http://news.disneylandparis.co.uk/calendar/index.xhtml?
Most Thursdays in March April both parks open at 10 and close at 7 or 8
I'd start at the studios, and do some of things that aren't at WDW, e.g. cinemagique, crush's coaster, stitch live (if its open) and maybe a couple to compare the difference with WDW eg ToT, moteurs stunt show (RnR is basically identical to WDW)
I'd head over to
Disneyland Park for lunch, and book in advance at Walts in main street as you'll get a great lunch and there's nothing like this at WDW.
There's info here on booking restaurants in advance
http://www.dlrpmagic.com/guides/practical/diningguide/
As some others have said I'd give a few of the rides that look the same as WDW a go as in some cases they're better than the WDW versions, eg BTM, PoC, Phantom manor, Small World, Space Mountain, Buzz and also make sure you see the parade and walk underneath the castle.
If you want to stay for supper I'd book the character buffet at Inventions in the Disneyland Hotel for after the parks close. If they're staying in Paris the DLH is right by the train station gto take you back.
Hope they have fun. The trick is to remember its not WDW, many rides are actually better, but the scale isn't quite the same so you need to appreciate it in its own right rather than keep comparing the two. On balance we actually prefer DLP but obviously you don't get quite the same weather and there's no Epcot, AK or water parks
Andy