Proper etiquette would call for the invited couple to have taught their children well enough to eat a little of what they don't like to be polite, and just eat the rest of the stuff they do like, it is acceptable however for the child to not eat portions of the meal if they cannot stand taht particular food, however commenting on the food in a negative way is very rude. Also the parents should be grateful for being invited to dinner and should arrive with a gift (a bottle of wine is usually a very good choice) so in that instance it is permissable for the invitee to ask what will be served (so they can choose an appropriate wine) any person invited to my home for dinner is expected to behave appropriately, or they will not be invited back. Also it is commonly expected for the invitee to place a call within a few days after the meal thanking the hostess again for the evening, in the south a letter is expected for formal dinners or by more well to do families. Phew mom would be proud hahaha, BTW this is paraphrased from my old ettiquette book.