Passports and Waterton NP
I believe there's an alternative to a full blown passport—there's a card that costs less. For four, passports + photos (if you have an AAA membership, they're free; some post offices will do it for a price) are something like $75 each for the passports. Though the backlog isn't as bad as it was, you might have to have expedited service to make sure to get anything by the end of July—and that's something like $50 extra/passport. I think up until June 30 (leave and come back now!!) you could do it with driver's license and birth certificate. There are lesser charges for minor children I think.
In other words it's expensive and in my view, dumb. I think it's an insult to Canada as well, our closest friends. But, no one in the gov't asked me.
You can get the forms from most post offices or online. Google something like "passports" and you can find the costs and all the other stuff you need to know.
There are RV restrictions on the Going to Sun Highway and you'd better check that out. If anyone has acrophobia in your family, this is the road that will make them freak out.
Waterton is different. I liked it better. Here's some photos to encourage you get passports someday: