That I can find, there aren’t any campgounds on the metro lines, since they are all pretty urban. There are, however, many campgrounds on the north or south shore that are close to downtown Montreal. Are you looking to tent or RV, because it will make a difference.
http://www.parcmontlaval.ca/Pages/contact_eng.html This place is in Laval, they are accessible by transit, but not metro.
http://www.sogerive.com/html/vr.asp is an RV stop, but no reservations possible. It is really close to the Longueuil Metro line, which gets you to Berri-UQAM on the Island, and from there it is really close to downtown.
I’m having a hell of a time googling campgrounds right now…! If you want to rely on transit your options are very limited. If you have a car… well, there are hundreds of places!
As for activities, it all depends on how long you’ll be here for. There is always shopping in Montreal, downtown, on the Main (St-Laurent street) or Avenue Mont-Royal being the most popular. The botanical gardens, the Biodome and the Biosphere can all be a lot of fun. Your kids might want to go to La Ronde, a local amusement park, located on Isle Ste-Hélène. Depending on when you come, there are all kinds of festivals throughout the summer, as well as concerts and shows at the various theatres and Place-des-Arts. The Old Port is very nice to visit as well.
While Montréal is the most bilingual area of Québec (other than perhaps the small villages of the Eastern Townships (Lennoxville)) you won’t have any trouble speaking/getting served in French pretty much anywhere. The West Island tends to be more English, as is Westmount, but they are less touristy anyways. Going off the island, in pretty much any direction except along the 20 towards Ontario, you won’t have to go far before hitting a town where most people don’t speak much English… even Longueuil and Boucherville, right across the river on the south side, are very very French. The Plateau is very French too.