I kinda did the same thing opening weekend, I was still hunting in 40 acres of pasture and had turned around to start back when I saw what I thought were 2 does on the other side of a ravine but ended up being a little buck chasing a doe, they were headed in my dirrection but further up the hill so I ran to intercept them and one ended up running up to me within 20 yards broadside,I reacted and shot with the 1st thought that it was a doe, only to see the forks as it turned and ran,I had 1 buck tag and 2 doe tags so it doesn' t matter, its good venison, but had I waited a second longer and seen the forks I probobly woulda let him go to get a little bigger.
Anyway, theres nothing wrong with shooting a fawn, seems all of our deer this year had twins, I purposly shot bambi this year and let the doe and the other fawn go, just cause we have so many tags available and there are so many of them out there, plus its some great meat, just takes a few of them to fill the freezer.