You'll all probably tell me it's perfectly safe, but I don't think I'd
be comfortable pushing such small stock through the router table with
just my fingers. Maybe that's my own inexperience, and maybe it's also
my wish to continue my 50+ years of playing piano, but I think I'd need
to find another way. I did make a set of panel doors a few years ago,
with 1x3 rails and stiles. I can't remember how I pushed the stock
through, but I doubt it was like that.

Pretty sure the easiest and safest way to make a Domino narrower is to run it against a file or piece of coarse sandpaper. Holding it with your hands.. Not much danger. Pretty sure I could take off 1/16" or 1/8" off the edge of a Domino with my file in less than a minute. Easy.