The OP said it was for his parents in their 80's with frequent leg pains and need to stop and rest. I doubt a 45 min connection is physically possible in those circumstances, never mind adding in customs delays etc.

I've done it in about 45 min on a BA-Westjet connection and when I got to the WS desk (they didn't think I'd make it to be able to check bags at the exit of the customs hall, so had to go landside) the check in agent looked at the clock and then at me and said "Can you run?".

He took care of my bag and I ran. It was evening so security wasn't at all busy and I made it to the gate halfway through boarding. No way I could have done that if I was in my 80's.