What tool for easily cutting cardboard is child-safe?
[Please see the revised and expanded version of this answer at Parenting Stack Exchange.]

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.