So what’s the right thing to say when a kid notices someone is fat in that semi-objective way kids sometimes do? You can say something like, “Yeah, it’s really cool that bodies are all different sizes,” if you don’t think a kid is putting a negative spin on it. The worst thing you can say is “Don’t say ‘fat.’” The caveat I’ll add is that if the lady at the grocery store hears you, there's the added layer of “It’s not OK to talk about other people’s bodies without their permission.”
Note: “Bodies are amazing: they come in different sizes, but we always ask permission before we talk about someone else’s body.”