The discovery dashed hopes that the 11-year-old girl might have been alive and clinging to a tree or rock through the night.
Emergency workers had already found the bodies of a 5-month-old, a 5-year-old and a 7-year-old. Three of the children were siblings and one was a cousin.
Two adults and three other children were able to escape. The horse that was pulling the buggy also survived.
Officials say the group was traveling in a downpour in the dark and may not have realized the creek had risen.