LaBelle's publicists and lawyer didn't immediately respond to Kevin and Roseanna Monk's lawsuit.
The couple live in a Manhattan building where the Grammy Award-winning singer stayed for a time while appearing in the Broadway musical "Fela!" last fall, said the Monks' lawyer, Samuel L. Davis.
He said LaBelle chastised Roseanna Monk for letting the toddler take some steps away from the mother as she grappled with some luggage and a car seat in the building's lobby on the afternoon of Nov. 11, 2010. After Monk scooped up the child and told LaBelle it was none of her business, the singer threw water on them from a bottle she was carrying and then launched into an obscenity-filled tirade, he said.
When the child started wailing, Roseanna Monk made a remark to LaBelle, and the singer charged at her and tried to hit her, Davis said.
The Monks filed a complaint with police; no arrests were made.
The lawsuit, which seeks unspecified damages, comes five months after a West Point cadet sued LaBelle over a Houston airport scuffle with her bodyguards. He said they attacked him for no reason in March 2010. She countersued the cadet, saying he tried to get into her limousine and was drunk and using racial slurs, which he denied.
Davis said Roseanna Monk had asked LaBelle for an apology and a donation to a children's cancer charity but was rebuffed. The Monks feel "someone's got to teach her even a diva can't attack and frighten and assault regular people in the building," their lawyer said.
LaBelle's singing career has spanned more than four decades, two Grammys and several hits, including the 1974 disco smash "Lady Marmalade."