I'm probably the wrong person to ask this question.
A child on the low end is not going to devalue the achievement of your badge in the class. How much harder is it for an autistic kid or someone with dyslexia to overcome and achieve your silly badge than a kid who functions normally?
The idea that a low functioning kid would somehow devalue a silly badge that is to show progress and learning is like saying the kid who starts on the 50 yard line should score the same touchdown as the kid who gets the ball at the goal line.
The best way to handle this is to make sure each kid is learning and moving closer to the goal.
I don't want to sound mean in this post but I am putting on my parent hat. I see my child in your post on the low end due to no fault of his own. He was born this way on the low end of the spectrum and while brilliant in many ways he doesn't always learn the same way other kids do.
I guess the thought of my son's teacher thinking he shouldn't be rewarded for learning because it would devalue the badge for the other kids makes my blood boil.