I would watch and wait overnight. Obviously, as I am certain you are already doing, brood artificially. Frankly, though, at that age if it isn't doing well in a few days, or takes a turn for the worse, I would cull it. From what I understand, breaks in bird bones don't heal the best, although if one would heal, you would think it would be a chick (baby). The recuperative powers of the young.
I had to cull a chick a few days ago. It wouldn't have had a good quality of life longterm. If you have to cull, do not feel bad. In the meantime, keep it comfortable and safe, it sounds like you are doing the right things by stabilizing the fracture. I have a litter of ferals caged right now in my shop. We trapped feral mother, and she gave birth before the spay/neuter clinic she was scheduled for. One somehow broke it's femur at about two weeks of age. In older cats, the vet said the position of the break would have required extensive orthopedic surgery (with screws) or euthanization, but this little fellow's bone started knitting within a week. He is now a month and walking fine.
Watch and wait. What worries me is the fact that the break was bad enough to break skin, and we all know a chicken coop isn't the cleanest place. If infection appears, I would try pen-g or baytril.