The SG clip on the Parkway would be great for a better belt fit. That'd be awesome. The Top Side and Harmony provide nearly universally great belt fits, and the blow molded boosters mean they're lighter than the Parkway, but that's fine if you want to take the Parkway. It means zero dollars spent for you. Much easier all around, and you know you're absolutely going to get a great belt fit. That's the number one most important thing.
I'd check my backpack and bring the booster to store on board. Nowadays, I just need my phone anyway. Or my kids need a drawing book and some coloring pencils. If it's that short of a flight on that we're on a Bombardier Dash 800, a sketchpad and new set of colored pencils or markers is fine. When we get to our connecting airport and we get the backpack back, then we can get to the rest of the goodies. But for that short of a trip, no biggie.
According to Transport Canada, you can bring on a child restraint.
https://travel.gc.ca/air/carry-on-baggage This page also says they don't count toward your maximum.
https://www.catsa-acsta.gc.ca/en/carry-or-checked Now, of course, space is an issue on smaller aircraft. So maximum or not, we cannot produce more space. That's when I'd say no problem, send off my backpack for the hour long or 90 minute long flight, and then put the booster in its spot. They cannot make you keep the backpack and check the booster. It's just a matter of space limitations. And I have. Even on larger planes, I've had my daughter have her backpack and booster and that's it, so she doesn't take up a lot of cabin space. I put her booster on top of my suitcase in the overhead, or under her feet on top of her backpack, so we don't use her booster to take up precious in cabin space. It has every right to be there, but that doesn't mean I want to piss off the cabin crew or my fellow passengers.
Also, $10 worth of chocolates for the crew goes an amazingly long way toward good will and looking the other way if it's just a slight issue. My daughter's booster didn't *quite* fit on top of my suitcase. I tried, but it wasn't 100% on top of mine. It was going to need to share space on top of someone else's suitcase (which isn't an issue, but I try). I don't know if the flight attendant even knew of the chocolates yet. But I tried. My daughter and I sat down, the booster got sorted, I fell asleep. When I woke up my daughter had a hand written thank you note, a bag of snacks, and some extra drawing stuff. And believe me, no one mentioned the booster. When I traveled along (to a carseat manufacturer, ironically), for my $12 bag of chocolates, I was offered an upgrade to the exit row. So, try bringing a little bag of chocolates. It may put the crew in a good mood.
Bring a few extra for you and your kids as well. It may help you guys as well.
Wendy