Ploppp:
Some people are fat and some people are not. It really has little to do with how much you eat...
This is true for some. But most fat people do eat more. And certainly all feedees who've gained weight on purpose eat a lot; some eat amazing amounts.
It's tricky talking to kids, though. Kids should not be encouraged to get fat; it's a major life decision that they are too young to make. (They may do it anyway, but it should not be because you encouraged it.) Of course I'm not saying you should fat-shame them either, just be fat-neutral. If they expressly ask why you're fat you could just say that you eat a lot. Or that you don't like dieting. I wouldn't tell them what to do unless they expressly ask: in that case I'd tell them they'd be better off eating moderately, at least until they grow up.