"It isn't polite to ask someone's weight, or to shame someone for their weight, or to compare two people's weight to their face, or make fun of people's weight behind their back. That's private, and people can have strong feelings about that. But we're not doing that here, are we?" -> Pivot into times when people who worry about numbers have to work with weight data, and that's okay -> Congratulate for being so thoughtful -> The endOh ok, definitely let the 9 year old girl know this! Of course we should have math problems in the form of comparing girls' weights. Because NASA. Who cares what an actual girl thinks.
It isn't hard. I say this as a father of two girls.