Your a/b is actually an a/b/c; don't know better/don't care/ don't have the energy might all be of a kind at first glance but it's a can't/won't/can't.
It's not a can't/can't/can't.
And that, right there, is the fundamental issue - externally, everyone here is arguing that fat people are fat because they can't or won't change (same diff) while internally, fat people are 100% all the time at "can't" or, more accurately, "have tried fucking everything and who the fuck do you think you are implying my can't is a won't".
And the answer is always some form of "someone calling you fat, fatty, what the fuck are you going to do about it."
Above, you're basically saying "these people can't or won't take care of their children." And while you might get agreement on "can't" (be it grudging, despairing or defiant), you will never ever ever convince someone they won't take care of their kids.
Especially when you're arguing that allowing them to look like their parents is child abuse.