The contents matter less than the rule: it only comes out on Friday, and it goes away again after.
A box available all week is a toy box. A box that appears at a specific moment and disappears after becomes part of how a child knows what day it is — which is the actual thing you are teaching.
Two: unbreakable things to carry and hand out. A soft toy that only exists on Friday. Napkins to put down.
Three: their own cup. A cover they decorated. Something to hand to each person.
Four and up: a job with steps — setting places, folding, a small book that lives in the box.
Six things you put away every week beats twenty you stop putting away by the third Friday. The ritual is the resetting as much as the opening.
The same moment every week, early enough that it is part of the getting-ready rather than a rescue at the table.
Rarely. Novelty works against you here — the comfort is in it being the same box.
Part of our guide to Shabbat with young kids.