We did Canada 3 yrs ago when DS was 15 and Dd 13, so slightly older than yours. We flew to Vancouver, had a week there (staying in a hotel but with a kitchen & lounge area, a godsend when you are jetlagged as you can cater yourself if you want). During that week we also went overnight to Vanc island, via coach/ferry, staying at Victoria. We arrived in Vanc the evening before their annual firework contest started, so caught 2 nights of that whch was wonderful (its Sat/weds/sat in late july)
We then hired a car and drove to whistler - a ski resort but wonderful in summer too, you can go up 1 mountain, then across to the other on a gondola. We then drove to the Rockies, staying at Lake Louise, Jasper and Banff and flew out of Calgary. I had been many years ago, but loved re-visiting and DH and kids loved it too.
We booked through trailfinders, and got comparative costs for car/hotels and a campervan, deciding the former was cheaper and made driving faster.
Should we go again, we would probably do Vanc/Island/Whistler OR the Rockies. However for a first trip and over 3 weeks, combining them was fine.
The other option is the East side, Toronto, Niagara etc.
If you do decide to go, feel free to PM me for ideas. My inlaws have also done similar trips to ours on at least 3 occasions, but staying in different places, so I can give you some route and accomodation ideas.