My experience is that explaining concepts like "body-positivity", "tolerance", and "common decency" (as in "the common decency to put your thoughts into polite words") are completely wasted on children and the mentally challenged.
The only viable solution here would be "better parenting" - and that is out of your hands.
DrSaturn2
One time, I wanted to make a chunky OC, but scrapped her entirely because the first reaction to her was "FAT FETISHIST", yeah
Slipnum
Something similar happened to me where I was making a more chunky version of my character Dumori and someone said I had a fat fetish. I just ignored them though and finished the drawing because I wanted to.