X2 what TwoCrows said, it is fine to leave her and the babies out for awhile, just keep an eye on them. Can she and the chicks get in and out of the hutch on their own, or do you have to lift them in and out?
Since they weren't brooding together, you do not want to put the new broody in the same hutch at the same time as Mama, mother hens can be very protective of their space, chicks and things like food and water.