The short answer is, you can't do anything about it.
I was always quite a skinny kid, right up until my early 20's when I started putting a bit of weight on. Then DD was born and life changed and it started going on even faster.
My Mum started on at me about it fairly early. "You've put on a bit of weight". "Do you think you need to drink a bit less? "Another takeaway?"
She was right, but she wasn't helpful. She wasn't telling me anything I didn't know already. I knew I was putting on weight, I just didn't care that much at that point, thought I could lose it when I needed to. All her mentioning it did was kick in my stubborn streak to keep eating crap, and damage our relationship a little.
By the point that it actually started bothering me, I'd gained too much and it felt like an insurmountable hill to climb. I made a couple of attempts at losing it, but nothing serious. And then two years ago I got diagnosed with type 2 diabetes. Mum isn't around any more, but I could still hear the "Told you so" from her echoing round my head.
Lost it all in about 6 months, put my diabetes into remission. I've still managed to do myself permanent damage in the mean time though.
In short, you can't do anything about it. You can keep mentioning it to them, but you won't be telling them anything they don't already know. They just don't care about it enough right now to change, and nothing you can say will change their mindset. All you'll do is harm your relationship with your child.