The cold air return in this house is in the floor, in the most-trafficked part of the house. It has a heavy grating over it.
My kids' toes are small enough to fit in the gaps, which is very painful when it happens. In the summer I cover the grating with a rug, but in the winter we have to keep it clear.
This is a rental, so relocating the cold air return isn't an option.
If the grate is easily removable, simply lift it up and wrap it in a thin, loose fiber fabric like a muslin, cheese cloth or nice smooth burlap. (not the rough course kind). Even a piece of regular fiberglass screen would do the same thing. This will look nice, let plenty of air through, keep excessive amounts of dirt out of duct work and help protect little piggies. Check out the fabric dept at Wally World, bet you can find what you need for a couple of bucks.
The problem of that grating is it has cells big enough to accomodate a kid toe. Just replace it with one with smaller cells or find a wire mesh with smaller cells and put it over the grating - something made of 3 millimeters steel wire will do, but you'll have to cut it to size and carefully process the wire endings along the perimeter.
Buy a rubber outdoor floor mat that has a hollow scrolled pattern and put over it. This works well for me as I have one too.
If you can remove it, you may consider cleaning and coating it with a plastic product (such as plastic-dip).
One thought because my 10yo daughter plays outside in the street a lot is the more your kids run around barefoot, the tougher their feet will be and the grating may not bother them as much when they step on it.
