Not sure what this procedure was when I was 6 year old girl?

I had a lot of UTIs when I was a little girl and I vividly remember one day my grandparents taking me to the children’s hospital in the city after my pediatrician recommended I go in for tests. The first one wasn’t so bad. It was just 2 ladies rubbing my belly with warm and cool gels. I asked to be alone for the 2nd part but they highly recommend my grandma be there to hold my hand. I remember laying on a bed screaming while a male and female told me they were putting a stick of soap in my urethra and that it might burn. Next thing I remember, my bladder was emptying even if I could only feel pressure and burning and there was a camera showing my bladder emptying. Any ideas of what this was?

NOBODY--ESPECIALLY IN A HOSPITAL, PUTS A STICK OF SOAP INTO THE URETHRA.

If you were very young when this happened, you probably misunderstood what someone told you. Perhaps a nurse (?) told you that were going to disinfect a tube that would be inserted into your bladder. Perhaps they used another word because you were so young or perhaps because you were so frightened you couldn't concentrate on what was being said or you later remembered incorrectly.

In any case, a disinfected tube will sting when inserted into a urethra, just as if it were soap. And perhaps your urethra was also infected, so no matter what was used to empty the bladder, it would have stung.

Sounds like it was probably a VCUG or another type of cystometry test.

Sounds like it could have been a Cystometrogram. Being catheterized would allow the bladder to empty without the sensation of peeing, and they have sensors that are able to measure and chart volume and pressure on a computer - I don't think they have a camera in the bladder.

Odd that procedure would be needed on a six year old,

From the internet: "These tests measure how well the bladder functions. They help diagnose problems related to urine control. These can be incontinence, difficulty emptying the bladder, overactive bladder, obstructions or frequent infections."