I've been trying to turn my kids into bookworms so naturally, we've been going to the Bibliothèque et Archives Nationales du Québec very often lately. I always go directly to the kids' section, so my review is based on that floor. Subscribing was very easy and free! We just provided them with proofs of residence and identity, then each one of them was given a subscriber's card.
The kids' area is awesome, they can watch documentaries and/or movies, listen to music, participate in activities such as video games workshops, currently they have a very interesting exhibit for kids about all aspects of Montreal (for the city's 375th anniversary), there's also a computer section and of course they have extensive books, movies and games collections for all ages, we always spend hours there!
If you're looking for a nice free activity to do with your kids this is definitely the place :)