Do you know when Rosario arrived in St. John Parish? Bennie was born in September of 1880. The census was probably completed around May of 1880. Rosario and Rosalie may have arrived in St. John Parish after the 1880 census was completed and moved to Kenner (that's in Jefferson Parish) before the 1900 census was taken. Unfortunately most of the 1890 census was lost in a fire, so we'll never know if they were in that one.
You could try searching the microfilm of the Jefferson Parish 1900 census.