why do people in quebec speak french

If I were you, with the mission of giving french-learning kids the experience of being immersed in the French language, I would make a point of going to the smaller, outlying towns around Quebec Province. In Montreal, so many people do speak perfect english, that you might not encounter the scene that would be memorable for kids.

Quebec City would be much better for that, but still better are the smaller towns. I can recall trying to order a pizza in Riviere-du-Loop from a woman who didn't speak a word of english, with me not five words of French.

Thankfully numbers and pictures made it reasonably easy. By the way, some of the small towns between Montreal and QC along the St. Lawrence River are picturesque places which also likely double as considerably french-speaking. I love the elaborate churches in each locale, even though I'm not religious!

Spend some time on the slower "highway" nearer to the north side of the river than the bigger "freeway" just north of it. Maybe stop in every little town to have some encounter with a french-speaker (gas stations, stores, fast-food places, etc. ).

I hope it works out for you! Make sure everyone has proper documentation for getting across the border and back.
A sign reading "Welcome" in French and English stands at the entrance to Morin-Heights, a tourist town in the Laurentian Mountains region of Quebec, Canada.

In Quebec, French must be the predominant language on signs, retail or food service employees always greet customers in French, and there are laws dictating whether parents can send their children to English or French school.