So: in one size order the trays will stack, reverse the order and they nest instead. They're deceptive because they look the same. Not tried it on younger kids yet, but my daughter likes it. Or just use it for storage!

Print in vase mode 0.8mm walls. Tested in PLA and it's fairly flexible, PETG might be better.

I've included the Fusion 360 archive file so you can change it to your heart's content: change the width/depth/height. Not tested other heights yet, the taper decreases as you increase the height, so it may affect how it fits. You can also change the taper angle (see sketch 1), just bear in mind, less taper is less clearance so they may not nest together. Could be remixed to be a sorter tray (I'm planning to at some point).