Grand Prairie caught my eye as there is a Grand Prairie, LA in Evangeline Parish. I am a native of Ville Platte, Evangeline Parish, LA. I have not done any research in Plaquemines Parish, but perhaps I can help with information on where to look.
You did not mention the birth years of Charles & Ellen Harris. If you have not already done so, read the 1880 census. The Jefferson County Library in Birmingham, AL has a subscription to Ancestry.com that includes the census. Ask your librarian if a library near you has this subscription.
Suggest your send your query to: Plaquemines Parish Genealogical Society, 203 Highway 23, South Buras, LA 70041. Enclose a stamped, self-addressed envelope for a quick reply.
The genealogical societies I belong to have published information such as cemetery records, etc. Perhaps one of the volunteers can do a look-up for you. Ask if there is a charge for this service.
Plaquemines Parish St. Thomas The Apostle Church (established 1843) Pointe a la Hache, LA. 70082 Registers: Marr. 6 Dec. 1864 Bapt. 4 Dec. 1864 Fun. 20 April 1865
If your grandparents were Catholic, the church may have records of sacramental events. Source: "A Guide to Church Parishes in Louisiana" by Rev. Donald J. Hebert
Plaquemine Parish was created in 1811 from Orleans Parish. Plaquemines Parish, Hwy 39, Pointe a la Hache, LA 70082-9999 Phone: 504-333-4343 (Area code may have changed. My reference is more than 10-years old) Parish Clerk has m rec from 1809, div, pro, civ ct & lnd rec from 1800. Call ahead and ask for fees. This would be a good way to learn if the town and its cemetery are covered with water. My references do not mention this. Source: The Handybook for Genealogists pub. by The Everton Publishers, Inc. c1991
Hope you and yours have a Blessed Christmas and a Happy New Year.