[I'm reposting my answer from your cardboard-cutting question at DIY.SE here, since it was closed there as off-topic.]
When I was the age to be cutting holes in cardboard boxes for fun, the tool I had was a serrated knife — I think it was sold as a very cheap steak knife. The blade was, basically as dull as a letter opener (and somewhat flexible), but the serrations made it quite effective at cutting cardboard like a saw, either neatly or roughly depending on the force applied. It did have an end pointy enough to require some care, but that was never a problem that I recall. It also makes a great noise when cutting a large panel, especially the side of a box, as the cardboard vibrates.
I don't have any specific product recommendations as this was a few decades ago, but I do want to emphasize that this is a dull knife — the serrations do the work. I think this may be the principle used by "won't break skin" kids' woodworking tool sets, also, but I haven't ever tried one of those.
(This is for corrugated cardboard. On non-corrugated cardboard, the serrated knife will still work but the cut will have a thicker edge of torn-up fibers.)
As to fastening cardboard, I always used clear package-sealing tape. (It's much better looking, and less messy, than duct tape.) You can't peel it off to reuse the cardboard without possibly stripping off a layer from the cardboard, but you can cut the tape and leave it on, if you don't mind the looks.