Just read a post about buying baker's sugar instead of using store bought ps with cornstarch in it. It's very simple to make your own. The easiest way for me is with a coffee grinder. You can grind up half a pound in just a couple of minutes. Just a few pulses and it's powdered! Not easy enough for large scale operations, but great for the hobbiest.
The quantity of cornstarch in powdered sugar is so low (2-3%) it's negligible especially of all you're doing is using it powder dusting. After reviewing the numbers (1 cup per deep) there's simply not enough volume of it, assuming they ate all of it, to have any effect on them. It's different if you're using it as a Fall feed in your syrup but then you should be using granular sugar for that anyway.