Which kinds of knives? If they are sharp enough, and the kid loses a finger or an eye, I suppose that is also a learning experience, but are we willing to let them pay that price?
Not dissimilar to what you'd do at home, except supervised closely with peers and integrated into the classroom.
Edit: I should mention that gross and fine motor control is a really big focal point in general. Like how writing things down has been shown to improve memory, other forms of tactile interaction increase engagement and learning.