How to deal...?

Andreas,
My children are vegetarian. Not fake (junk-food) vegetarian, but real organic whole-grain/vegetables/fruits kind.

Every time I'm not looking, my in-laws stuff my kids with processed meats, burgers, deep fried foods, candy, sodas, cookies, etc.
Kids are still too young to make their own decisions. And, unfortunately, junk food tastes better than the real food at home.

I'm lost. I can deal with people who just don't cooperate, but I have no idea how to protect my children from their grandparents who have been consistently and viciously damaging my children health for years. Negotiations are fruitless; they cause nothing but conflicts.

Please advise.
Thank you.