Keyshia Cole trusted a 12-year-old amateur singer to help her belt out a song at a post-Emmys bash ... and the result was simply awesome.

Keyshia performed her song, "Trust," Sunday night at the Golden Soiree at Viva Hollywood -- and she invited a 12-year-old named Serenity, who happens to be in the Make-A-Wish Foundation.

We're told Serenity's a huge fan of Keyshia's and had requested to sing with her -- a wish that was granted by Jimmy Iovine's son, Jamie ... who's close pals with Keyshia.

As you can see, the duo was a hit with the crowd ... and Keyshia even gave a special shout-out to Serenity and Jamie afterward.