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So sorry! Usually the chick will stay under mama hen and she won't leave the nest until a day or so after hatch, if she hears them peeping. Of course, in this heat, the chick may not have stayed under mama hen, but instead gotten on top of her back and fallen out of the nest. If that happened, mama hen wouldn't go after the chick if she hears peeping from the rest of the eggs, so the chick would have been unprotected from the other hens.
Once that first chick has hatched, you can move mama hen and her chick to a cage or pet carrier as Greybear suggested. She will stay with the rest of the eggs and her chick that way and can protect her chicks. I have had mixed results trying to move a hen before the first chick hatched - if she wasn't "in the hatching zone" she didn't always stay on the nest until the eggs hatched if she had other options about where to go.