Unstructured play is very common, but at DS' daycare the kids are still kept in age groups - I would not be happy that she is with kids that are so much older, or with the carers inappropriate action, and I would report it to the director of the centre, or whoever is in charge.
AsK to see thier policy on behaviour (bad/challenging I cant remember the right term but they should have one) i bet it does say to tell the child being bullied to toughen up!