It happens so often where a relative or mom friend will say things like, "Oh, I need to lose this belly I gained over winter," or, "I shouldn't eat that because I'll balloon up." When your child hears comments like that, they'll think it's OK to talk badly about ourselves, and then they'll start to criticize their own bodies or think they need to restrict the food they eat to lose weight.
When you hear comments like that, if you feel bold enough, you could say something like, "In our family, we don't say negative things about our bodies." Or you can counter their statement with something lighthearted or a compliment like, "Your body is perfect just the way it is," or, "You can trust your body to know what and how much food it needs," and turn to your children and say, "Right?" Or Manley suggested having the person model what you teach your kids and say, "Name one thing you like about your body or one thing you're good at."
Depending on how comfortable you feel around the person making the comment, you can always pull the person aside and ask that they not make comments like that around your children, Cadillac suggested. Black also said to follow up with your kids after the interaction and talk to them about how some people feel badly about their weight or how they look, but that it's the kind of person they are that's most important.