Isn't it more likely that the chick came from a small Jersey Giant or Minorca egg or some other breed that isn't coming to anybodies mind, than somebody handing out a little existent breed's egg?
How big is it compared to a silkie chick of the same age?
Just a thought, but the australorp/extended black chicks I've seen all have much more white on them and lose it as they mature. This looks more like a black sex link chick, which ends up with less black/other colors like red/gold around the neck, etc.
If the person just *gave* your child an egg, it's very possible it was a mixed breed, possibly black sex-linked. There's really NO way to know without knowing the donor's flock.