There is an easy answer to prevent that::::: When our boys were growing up, we had a no-eating-in-anyplace-but-the-kitchen-or-dining-room rule. Period. The exception was when we (or they) were having a party.
If you must install a television in the kitchen to make this work, so be it. (We also limited the time our boys could spend watching television on school nights, but that is another topic. Actually, our household wasn't big on watching television at all, just because there was always something more interesting to do.) But right after school they wanted a snack and to watch a favorite 30 minute program while they relaxed (school is stressful to kids, just like a job is to an adult). Then they went out to play for a while before supper, homework, bath, and bed. I was a working mother, but as a teacher I was able to be with them in the afternoons when school was out, and obviously that helped..
When our grandchildren came along they were allowed to carry around sippy cups of juice by my DILs, and this really caused a problem with carpet stains. The grandchildren stay at our house often (which we love and they love), and I began to allow them to bring a sippy cup in the den **but with only water in it, and they put it on a large cork coaster.** You'd be surprised that an 18-month-old can adapt quite quickly to this system. They enjoy water and there is no complaining. Children need water, and they often don't drink enough of it. At the same time, I put out nutricious snacks (their favorite ones) on the kitchen table, and the grandchildren know to eat their snacks and drink their juice while in the kitchen and not to bring them into the den. We are lucky in that our kitchen opens completely into the den and we can thus supervise them and they can visit with us and even watch television when eating snacks. Now when their parents are with them at our house, the children follow the system that their parents see is already working fine.
I guess I am old fashioned in regard to setting expectations and standards that help the house to run well, but it worked for us for all these years. I think that young parents don't realize that they can set rules and have them followed. Our boys have thanked us many times for the upbringing they had. If you set basic parameters, then the children can have great freedom within those parameters, and the household can still be relaxed and loving. I must add that being consistent with the standards takes a lot of energy on the part of the parent, but in the long run it is worth it..
Just my opinion that I thought I'd share. I know many will disagree---so many ways of raising children seem to work well, and it is an individual choice of the parents. As they say, "Your mileage may vary."
Well, I rambled and got off the topic of carpet stains.. My apologies---and appreciation for your patience.