I know Landry Academy offers several, my 9th grader Makayla just finished Graphic Art using Adobe Illustrator. However I would be very careful to pay attention to the grade levels they suggest the course is for. She did a high school course and it was very much challenging. Great for her, but would be totally overwhelming for a 6th grader.
On the other hand, consider searching on Amazon with the words “graphic design kids”. I just did and the first several books that popped up look pretty good. Go and Maker Projects for Kids Who Love Graphic Design stood out to me.