Knifes are not only for cooking, and they are just fun to play around with, when I was a kid we built makeshift shelters in the forest near the town with knifes and hammer and even a saw.
The problem is not the knife, but the children not being property disciplined on how to use them. Restrictions on this common of an item will never work because even if you make the perfect system so no child can ever buy one, they will just get one from home or use a broken piece of glass.
There are a lot of dangerous things in our world, restricting the access to them is just impossible and frankly stupid.
Yeah it makes sense, I do agree knifes should not be sold to children.