Which is ruder - to let people know there are certain foods that your children can't/won't eat, or to let people prepare food that they can't/won't eat in ignorance, and then see the children either refuse to eat it, or push it round the plate and take an hour to swallow 3 mouthfuls because they're being forced to eat something they hate?
If I offer to feed someone, as a hostess, I want to give them something that they will want to eat. If they don't warn me that their child can't/won't eat something, and I make that something, I'm going to be upset, they're going to be upset and ALL OF THAT could have been avoided by getting the information up front.
Avoidance of upset is far better manners, IMO, than creating a situation through a misplaced sense of "what one should do".
And yes, I was forced to eat any amount of disgusting food when I was young - until I was actually sick. After that, avoidance tactics were more acceptable. And no, I couldn't help it.