Graces' most famous version of this song is the one she did with Ingrid Michaelson.
Rockin’ around the Christmas tree features Grace Vanderwaal who is a mighty ball of talent, she is incredible. I reached out to her on Twitter actually, and I said, “would you ever want to sing on a holiday record of mine, and she said “yes, yes of course”, it was. very sweet and yeah, we go together in the studio in Brooklyn and she knocked it out in like 445 minutes, she a total pro, she’s so talented and the song is kind of one of my favorites, … and I love Grace, so I’m super happy about it. (Ingrid Michaelson: Track by Track: “Rockin' Around the Christmas Tree” (Feat. Grace VanderWaal), Published Oct. 12, 2018)
“Grace has such an old soul voice,” Michaelson said in a statement. “Her sound is so unique, I knew she would bring warmth and energy to the song.” (Ingrid Michaelson, interview with Billboard, 10 Dec 2018)
“Grace is just like – she’s fourteen years old – but she’s so - she’s like a very old soul, and her voice is just like, it comes out of her mouth the way you hear it, and that’s – when I say that, a lot of times with a singer, you know, when you really hear them singing, what you hear on the record is not what they really sound like, and she is like pre-autotuned, pre-mixed and mastered like in her vocal cords and then it just comes out and it sounds perfect.” (Ingrid Michaelson in Q&A at Barnes & Noble 15 Nov 2018. Video is in the list of related media)
"My mom always plays this song on Christmas morning before my brother and sister and I start digging into our gifts under the tree. What great memories this song brings." (Grace in an Interview with USA Today Dec 16, 2016)
You can read more about Grace's relation to Ingrid Michaelson on the Artistpage for Ingrid.