A fun way to teach line to grades K-3 is the Crazy Hair Day project. I start by reading the book “Crazy Hair Day” by Barney Salzburg, (or I let Salzburg read the story to my students via the magic of YouTube).
Then, I instruct students to draw half a face (or, for k-1, show them how to draw it, step-by-step) and let them make “crazy hair” by drawing as many different kinds of lines as they can come up with from the head to the edge of the paper.
I always illustrate the difference between intentional (straight, curly, thick, thin, zigzag, etc…) and non-intentional (scribbling, without thinking of each line you draw) lines. Intentional lines are good, but scribbling, not so much.
The results are fun, and students love this lesson!