Help me change my wife's mind about my daughter's diet

My daughter is a young 20 months old. My wife and her side of the family seem to think that Sugar is perfectly all right to serve a child on a daily basis. Sugar served in the form of candy and chocolate. My in laws brought over some donuts one day, they always bring over some type of sweets, and I said in my frustration great something else to make her fussy. My mother in law said "that is a myth that Sugar makes babies fussy". Needless to say they are quite stubborn.

I consider myself pretty knowledegable about diet. I am just looking at a way to make them understand.

I don't want to talk poorly about my wife or her side of the family. I am just really concerned about my daughter health and well being. My wife is resisting because she doesn't want to change the way that she eats. She is compulsive and picky eater.

I don't want to be a preacher. I don't remind my wife that what she is eating is bad. I just wish there was some way that she could understand.