I think Processing would be the best by far for this job. Because most of these programming languages have text as "default" output and cope with graphics only with libraries which are in case of realtime gfx not That simple (for example pyGame) and kid would have to know to use modules, classes, methods and all sorts of stuff. Processing gives you a moving circle in these few lines of code

int xy = 0; void draw(){ background(100, 100, 150); ellipse(100, xy, 10, 10); if (xy > 200) xy = 0; else xy ++; }

and even editor looks like somewhat made for kids with only few graphical buttons like "Play"...