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Would that happen even if you laminate it + stick it in a dark place?

In my experience yes. Old receipt roles fade even stored in a warehouse. I would try that anyway, as an experiment.

A cheap thermal printer for kids is around 10-20 USD (got one for such price with 10 rolls of paper, half of which is sticky paper), has bluetooth, usb and battery. Fits in a pocket. Would be fun to stick such qr code inside some not important book left lying around.

I would caution about giving children receipt paper. It may contain a lot of BPA (or replacements).

https://www.smh.com.au/business/consumer-affairs/controversi...

The laminating process would probably blacken the entire paper

There are laminators that don't use heat ;)

Yes I found this out the hard way just last week - thought I was being pretty clever with my inside-the-box packing label.

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