Weaving on a cardboard loom is a great craft for kids. They can make a bookmark, teddy bear blanket, pincushion, small purse, etc. with just cardboard, scissors, and yarn. (A tapestry needle helps, too.)

The concept of weaving is simple- the yarn goes under, over, under, over... and then the next row does the opposite. To speed up this process, heddle bars were invented to raise every even thread when rotated one way, and every odd thread when rotated the other way.

Heddle bars make weaving much faster and less tedious. Once a kid masters the pattern of weaving, learning to weave with a heddle bar could be a fun next step and a lesson in how tools can make our work so much more efficient.

The STL posted above is ideal for simple yarn weaving, but the .scad file is super customizable. Teens and adults may want to print a longer bar with narrower, closer-set grooves to make larger pieces with finer materials like embroidery floss, or print a custom bar to fit an existing loom or a specific project idea.

Here is a link to a tutorial on making and using cardboard looms: https://www.auntannie.com/Textiles/WovenBookmarks/

update 1/8/2023
I updated the code in the .scad file so that it is much more amenable to changing the parameters. I also changed the formatting to make it work properly with Thingiverse's customizer app. The main values you may want to change in order to accommodate a different weaving project are groove_width and n_grooves.

groove_width = 2; //[0.5:0.1:8]
// width of the grooves and depth of the shallow grooves. Needs are determined by the thickness of the yarn. 2 mm is good for average knitting yarn.

n_grooves = 20; //[3:1:148]
// number of slots for the warp strands

shed_width = 8; //[3:30]
// the size of the space created between the raised and lowered strands of the warp (the "shed"). A larger shuttle requires a greater shed width. If the shed width was zero, "shallow" and "deep" grooves would be equal in size (and useless).

tab_length = 8; //[0:2:30]
// length of the tabs on the ends, for easy rotation. Bigger thumbs need bigger tabs.

tab_thickness = 2; //[1:0.5:5]
// thickness of the thumb tabs; limited only by sturdiness. I found 2 mm was plenty solid.

height = 6; //[3:12]
// height of the rectangular prism, not including rounded top

shallowness = 1.14; //[1:0.1:3]
// Curve parameter for rounded top. Set to 1.0 to make the curve as pronounced as possible. Increase to make it more shallow. This belly helps keep the strands from slipping into the wrong grooves when the heddle bar is rotated.