Is there an appropriate age to tell your son/daughter about Gehinnom? Is it ever appropriate to tell your child that if they don’t eat Kosher or do something else in conflict with the Torah that they are punished there?
Children should be taught that the same way H-shem gives reward for mitzvos, He also punishes for doing aveiros, and that reshaim go to gehinnom. They can also be told that H-shem also punishes in this world. I don’t think that a child should be told outright that he is going to gehinnom if he doesn’t eat kosher. First of all, as a child he is patur, and if anything the parent is the one punished for the child’s aveiros. Secondly, although the child has to know that this world is not a free for all, to do whatever we want whenever we want, nevertheless, the way we introduce the mitzvos and are mechanich them sould be in a positive manner, and the focus should be on the positive.