Ideally, you want to shoot with a translucent blinder, not an opaque one. Your pupils dilate relative to available light, but you do not have the ability to dilate them independently. If one sees light and one sees dark, they both dilate to somewhere in-between. So if you have a dark blinder, it will cause your shooting eye to over dilate.
Unless you have a head injury! ;)~
I use a blinder, because I'm shooting with my non-dominant eye. Blinders can be cobbled from one gallon plastic milk jugs, just keep an eye on the size limits in place by some sanctioning bodies. If the kid is already shooting well without it, and hasn't mentioned "seeing double", then I don't know if I'd complicate things by adding a blinder.