"Your body needs to be as healthy as possible so that you can do the things you want to do in the world," is a great place to start. A child should understand that their body is not an ornament, an object or an invitation to commentary.
"You can fill their toolboxes with some all-purpose deflections: "I'm lucky to be strong," they can try saying, or "These legs definitely get me where I'm trying to go." Or some redirections: "Did I show you my muskrat diorama?" or "Is there coconut in this pie crust?"
"If you want to communicate directly with your relatives, you can explain to them in private that you're trying not to focus on the kid
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