O.J. Simpson was surrounded by family in the final days before his death, and that includes all of his living children -- yes, even the 2 youngest he had with Nicole Brown Simpson.

Sources with direct knowledge tell TMZ ... everyone who came to visit O.J. at his home in the days before he passed had to sign NDAs for his privacy -- that went for friends, family and even medical personnel who were coming into contact with him while he was under hospice care.

We're told on Friday, a medical professional checked on O.J. and notified those around him that he was starting to transition -- and that's when everyone came flocking to his bedside.

It's unclear exactly when his 4 children arrived, but by the end ... they were all there as he died peacefully. The family members included his 2 children with Nicole -- Sydney and Justin -- and his 2 older ones with Marguerite Whitley ... Jason and Arnelle.

It might shock some to hear that Justin and Sydney were in the mix ... as their mother was brutally murdered, allegedly by their father.

While Simpson was acquitted of the murder charges, he was later found civilly liable for Nicole's death and Ron Goldman's. In the aftermath of all that, O.J. raised Justin and Sydney.

They've never spoken publicly about the whole saga, so we don't know how they feel about it. Nor its it clear how close of a relationship they maintained with O.J. in their adulthood. Sydney is now 38, and Justin is 35.

Aside from family members ... we're told dozens of friends went to visit Simpson in his final days. As we reported, his remaining close friends flew in to say their goodbyes.

Our sources say somewhere between 30 to 50 people -- consisting of friends and other family -- saw O.J. in person. They all signed the NDAs, and no phones were allowed in the room with him.

Like we told you, he was in very poor health before dying ... and couldn't talk much. Still, these folks got some face time with him and said their farewells.