The only thing better than starring on a TV show with music legend Gladys Knight ... is getting singing lessons from her.
Knight -- who co-stars on "The First Family" as the President's mom -- was filming a scene with her TV "grandchildren", which required them to sing ... on key. But the youngins couldn't hit the notes.
In the video ... Knight gave her young pips -- Khylin Rhambo, Yara Shahidi, Sayeed Shahidi, and Layla Crawford -- a crash course in singing, with a little tune called "This Little Light Of Mine."
It totally paid off (duh it's Gladys) ... because the kids nailed it the very next try.