Hello ! I'm a Montrealer, so I'll try to help . The weather is very unpredictable, so I would bring jeans and a jacket. Mid April usually sees the first signs of spring, so you might be lucky and have sunny days and weather in the 40s or 50s.
As for activities, I would reccomend a walk in old Montreal, there are tons of good restaurants, including Olive et Gourmando, as well as SoupeSoup, a chain of cafes serving soups/salad/sandwiches, very fresh and delicious. If you want to eat like a Quebecois, you must have poutine. Some people like the fancy kinds (there are tons of options for those), but I like the standard from la Belle Province, a greasy spoon chain. You can also try st-Hubert, which is a rotisserie chain. They have a kids menu and they are truly comfort food (fries and sauce, mmmm).
Musée Pointe a Callière is our archeology museum, it's very interesting and my kids loved their tour.Biodome is fun as well, but it's a little outside of the main attractions, so I would consider a half day to be necessary.