I solved this by then introducing the kids to slices in a pie. Visually, the kids can see that 1/3 of a pie plus 1/3 of a pie is not the same as 2/6. Fractions are often introduced as "parts of a set" which is great for the first day. It's easy for kids to see and understand, but kids are more familiar with trying to slice the birthday cake for all the kids at the party. After the kids realize there are two kinds of fractions, we spend time at each problem thinking through is this a set (which we'll return to in ratios) or a whole (which can't be expanded) and visually representing fractions both ways to check their answers. Adding units is absolutely important, but when I teach it, that's what I add in fourth or fifth grade when I'm teaching them to reduce.