I've had hens raise chicks with the rest of the flock from day 1 with mixed results. One of my previous flocks were awful with new chicks and I lost a few, but my current flock is amazing and I've had no problems at all. As the above poster pointed out, a small chick is fragile, but a broody hen that is fully into mothering is a force to be reckoned with too. A good broody will protect her chick(s) against anything, be it other flock members or predators. (I've seen a broody hen beat up a hawk that was after her chicks once).
Ideally you should reintroduce mom with her chick to the flock some time before she weans it, which should be when the chick is around 6-7 weeks old. This way the flock will get to know and accept the chick and the chick will be fully integrated by the time mom loses interest and leaves it to fend for itself. So don't put it off too long. Good luck!