I don't do 70 minute connections in YYZ even when I don't have a customs or security re-clear, and I'm flying by myself with carry-on. You've got small children, and have to clear customs although not security, and you don't need to worry about your bags.
Check the math:
- 70 minutes really means 55 minutes since you need to be at the gate 15 minutes before the flight closes. Even if you are able to cut it a bit tighter than that they will close the flight some time 15 minutes or closer to departure.
- Out of those 55 minutes:
- It could take 10 minutes to deplane if you have small kids and are further back in the plane, especially if you have a stroller
- Then you need to walk from your F gate to domestic transfer customs, let's leave 5 minutes for that, but it could be longer depending on what gate you get
- Then you need to clear transfer customs; this can take anywhere from 1 minute to 20 minutes depending on the lineup. Let's say 10 minutes. NEXUS as pointed out above is irrelevant, there are no NEXUS machines at US-Can transfer customs in YYZ.
- Then you need to go downstairs (probably using the elevator with small kids), board the bus to the D gates, wait for it to leave and then drive over to the D gates. This could easily be 10 minutes.
- Then you need to take the elevator up and walk down to your D gate, which to YVR is probably around D40 down the end of the D3x/4x domestic pier. Another 5 minutes.
- So 55 mins - (10+5+10+10+5) = 15 mins. You have 15 minutes of slack time. Your flight arrives just a little bit late, you get stuck waiting for a gate crew to deplane or fetch a stroller, or the customs lineup is a mess for whatever reason and those 15 minutes can evaporate quickly. And it isn't like sprinting through the terminal with small kids is an option.
So technically yes, you can make it. Maybe your flight will even be early! Both of my AC flights today were
Or maybe the YYZ-YVR will be delayed, giving you more time. I would not count on that though.
Thus, your chances of misconnecting are high. I would make sure you understand the alternate/next flights and be confident there is room on them. Air Canada will reprotect you on to another flight, but it may not be the next one that would still give you a chance of getting there on time. And even if you make it the stress of this connection is going to be weighing on you the entire time.
Only you can decide whether those risks are worth taking. Just be aware that they are high, personally higher that I would take.