The Canadian Prime Minister was a guest on a Facebook Watch show called "New Mom, Who Dis?" where he was asked questions by a small group of school children ... including a couple uncomfortable questions from black twin girls.

One of the twins first asks Trudeau why he painted his face brown, and he responds by admitting it was the wrong thing to do because it hurt people ... and he's learned that now.

But, then the other twin asks a follow-up -- "But, did you paint your nose and your hands brown?"

Trudeau says "yes" and repeats it was wrong and hurtful, and apologizes to all the kids and people he hurt. He also says he's used this as a teaching moment for his own kids ... urging them to be respectful toward each other and not tease or discriminate.

As you know ... the scandal revolves around a 2001 yearbook photo that surfaced showing the PM in blackface when he was 29 years old. After the initial report surfaced, 2 other instances surfaced where Trudeau wore black face as part of a costume.

He's been slammed for it and is trying to make amends ... in particular when he sparred with black boxing champ Ali Nestor as he battles to get re-elected.