As the concept of fairness is innate in primates including humans in the 4th grade I wouldn't stop and correct the students but try and re-frame the problem in terms of a problem where fairness was involved and then immediately the ratios and the units would come into more clear focus. As an example I might say that if I promised to give you 1/3 of the pizza as a reward for cleaning your room. And then how much should you get if I had 1 pizza. How about 2 pizzas? Things like that.