It's definitely worth emphasizing that politeness does not extend to eating something that would make you sick (or, generally, letting yourself be forced to do anything that would be truly harmful to you!) But learning how to refuse politely is an important skill, even if what you're refusing is an allergen or something - it will, if nothing else, generally make it much more likely your refusal will be accepted without escalating, if you learn how to make it in a polite way. It can also make it much easier to refuse if you have a polite script to do it with rather than scrambling with awkwardness in the moment. And it's really useful to think about the line between "I have to refuse this no matter what", "I could do this but I really really don't want to do this and anyone who tries to make me is being rude themself" and "I don't want to do this but it's something I'm willing to do to make a friend happy or smooth a social interaction". The more you know where your lines are in advance, the less likely you are to be forced into choosing between rudeness and something that will harm you in the moment.
In the example in the letter, a kid who as a guest knows how to linger over a meal and then say "Oh dear, this is delicious but if I eat any more I will be sick" in the locally socially acceptable way is likely to get a lot less friction from the adults than a kid who eats three bites and then pushes their plate away and says "I don't want it yuck."
And ~9 is exactly the developmental age to be working on the more advanced versions of all of those. (Kids in general often need help figuring out the line between "I don't want to but I will for a good reason" and "It's not worth it for any reason", and a lot of adults tend to swing way too far one way or the other with younger kids and then expect them to just know the difference when they get older.)
(Should you have to be polite and gracious about saying "I can't eat that, it will kill me?" when you're a guest? If the choice is eat it or be rude obviously be rude! And sometimes you're at a point where being polite is beyond your capacity and it's nobody's fault. But you should always be polite and gracious when you have the option, and having a medical reason to refuse doesn't remove the option to be polite.)
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In the example in the letter, a kid who as a guest knows how to linger over a meal and then say "Oh dear, this is delicious but if I eat any more I will be sick" in the locally socially acceptable way is likely to get a lot less friction from the adults than a kid who eats three bites and then pushes their plate away and says "I don't want it yuck."
And ~9 is exactly the developmental age to be working on the more advanced versions of all of those. (Kids in general often need help figuring out the line between "I don't want to but I will for a good reason" and "It's not worth it for any reason", and a lot of adults tend to swing way too far one way or the other with younger kids and then expect them to just know the difference when they get older.)
(Should you have to be polite and gracious about saying "I can't eat that, it will kill me?" when you're a guest? If the choice is eat it or be rude obviously be rude! And sometimes you're at a point where being polite is beyond your capacity and it's nobody's fault. But you should always be polite and gracious when you have the option, and having a medical reason to refuse doesn't remove the option to be polite.)