If I lived in Canada (and given the weakness of the Canadian dollar versus the USA dollar), and if I had kids (of almost any age), and I wanted to stay cool, and I had several weeks in the summmer, I would head for the Canadian Rockies, period. Glacier National Park in the US is very nice, and not far from Canada, but Glacier National Park in Canada is also great! And of course Banff, and Jasper, and Canmore, and do NOT miss Peter Lougheed Provincial Park (which would be a national park in any other area of the world).
The best advice I can give you, though, is don't do what I did -- spend too much time driving with kids. There is a good reason that the English language includes the cliche "you are driving me crazy!" Let the kids have lots of time doing kid things -- playing in the mud, throwing rocks into streams, gathering sticks for the campfire and then tossing them onto the fire, etc.
The southwestern US is great in September -- too hot in July and August.