"kneeling down" is correct and usable in written English.
You can use it when you are describing someone who is prostrate in a supplicating or humble way. For example: "The man lowered his head and knelt down in prayer."
"Kneeling down, performing some trick.
"I remember kneeling down and absolutely sobbing into the carpet.
She came right over to me, kneeling down at my side without a word.
Kneeling down, you felt transported — you could almost hear rifle shots and Dietrich singing.
A devout Muslim recalls kneeling down and praying with his Christian cousins.
Another friend broke her ankle kneeling down to plug in a telephone.
Seconds later I was kneeling down next to her saying her name.
One witness said the gunmen shot a child who was kneeling down and begging for mercy.
They were kneeling down while sharing an eyepiece when Mr. Launie began what Dr. Schechner described as a "preamble".
Thomas Ryan was kneeling down, whispering a prayer into the ears of another dying man when he was himself shot.
She was kneeling down in front of him and she could feel him examining her the way he often did.
