One more thing to consider: I've never transited SCL before - is a visa required for international connections? A cursory search turns up U.S. citizens must pay $100 for a visa, but I'm not sure if this is required if only connecting to another flight.

As far as Atlanta, yes, your daughter will have to claim her baggage and carry it through customs. Afterwards she will drop the bag off again as it will already be tagged to her final destination.