I agree that you can't control what your children eat at other people's house. I suggest that when their overall diet is healthy they won't be harmed by eating the "bad" stuff elsewhere.
I put "bad" stuff in quotes, not because I don't agree with your choices, but because there is no research that shows that eating these foods some of the time will have a negative effect on children's health. Actually, there is no research that shows that they are a negative influence on diets unless they are a major part of their diet. Remember research is done with animals and they are fed a whole lot more of these substances per their body weight than we'd ever feed our children.
My granddaughter is allergic to peanuts. Because eating peanuts can have an immediate life threatening affect she has been taught to never eat anything containing peanuts. She's very conscientious about asking about the ingredients of what she is giving.
My granddaughter's curriculum at school includes learning about healthy diets. She's 10 now and asks me or makes comments about other questionable ingredients such as the two you mentioned. I suggest that the best way to reduce the amount of these ingredients that your children eat is to teach them in several different ways why you don't want them to eat them. Co-opt them into the decision to not eat them.
I also suggest that you allow a small amount of chips, cookies, sodas while choosing the healthiest of those. Talk with your children about why you chose this product over that product. When you deprive children of many foods they do tend to binge when they are able to have them. What is key is education and reasonable educated exposure.
For example: My grandchildren are told that they can have soda when they are with someone who serves soda. They have been taught that some sodas are healthier than others. I don't know if there allowed choices are better but I go along with them because their mother has told me what she has told her children. My point is that she is giving them an alternative rather than saying no to all sodas.
I appreciate your choice about healthy foods. I'm saying that when you give your children a healthy diet at home, teach them why you've made those choices until they are able to make those choices for themselves they will be OK. No need to be rigid when they're playing elsewhere and no need to allow that to make you crazy.