For building large structures like this, we've found Makedo's are great. https://www.make.do/collections/all-products

The screws and screwdriver make it super easy to connect the cardboard together, and I've found the fit to be surprisingly sturdy. It's reusable and pretty quick to remove from the cardboard once you're ready to take everything down.

The saw, in my opinion, doesn't cut that well. Possibly still better than having the kids use large scissors or a box cutter though.