Using Magic: The Gathering boxes for younger children and teenagers might be a more palatable way to introduce the zettelkasten method of note taking to them, particularly when it's a game in which they have a pre-existing interest.

https://www.amazon.com/mtg-deck-box/s?k=mtg+deck+box

First reference I've seen for someone suggesting using half size 3 x 5" index cards so that they could use commercially available Magic: The Gathering (MTG) boxes.

Oxford and possibly other manufacturers already make 1/2 size 3 x 5" index cards.