This is a hotly debated topic and without making a big argument here I will add that shorts in the main dining room is touchy. Have people worn shorts before in the MDR? Yes. Will you get kicked out? Probably not.
Personally, based on the dining room dress codes, I would not wear shorts and I do not think wearing pants/slacks is that much of an issue to wear instead BUT how you or your family dresses has no bearing on me or anyone in the MDR.