I remember a story from a 1980s children's magazine (probably one of my younger brothers' subscriptions, so likely the late 1980s) about two friends—a boy and a girl, who break a chocolate bar in half, then start arguing about who got the bigger piece. When they put them close together to compare, a purple mist starts coming out of the narrow space between the chocolate chunks, which surrounds them and teleports them to another world.
In this other world, they find themselves in the office of a king. (They know he's a king because of how he's dressed.) The king has some productivity problems, because everything in his office is the same size. I specifically remember that his phone occupies the entire desktop, so he is not able to get any work done without removing his phone. The kids explain that things can be made in different sizes, and they hold up the two chocolate pieces to show they are unequal.
Once again, the mist comes out, and this time it teleports them home. They experiment and find that they can get the mist to start appearing again if they bring the pieces close together again, but they separate them immediately, before the mist spreads. Knowing they can continue to use the magic candy pieces, they decide not to eat them, but they also think they have had enough for one day and so go looking for another snack.
This was probably the same magazine that published these other stories I have asked about: Seeking a children's story about living in a horrible "intelligent" house and Children's magazine story about cat turned into a cyborg