This could be an ethnic thing also. The food from some countries, India for example, has different spices and can cause different body odors. Their sweat just smells different. I remember reading about military persons stationed away from home saying that the native people in the country they were deployed in thought Americans smelled funny or bad. What they found out was that because Amerricans traditionally eat more meat our sweat smelled foul to them, even though the person had showered before going out.
It could also be dirty clothing. Some people just don't do laundry often enough, they seem to think that because they took a bath/shower and put on clean clothes they must still clean the next day and the next and the next.
It could also be bad teeth, UTI, ect.
I would mention it to the parents casually. 'We love having Susie in class she is bright and friendly ect ect. But we have noticed that she seems to have a body odor and the kids are now noticing it. Susie is so bright and friendly I want her to be accepted and have lots of friends."
I know the kids are little but I would also suggest that you do a lesson on hygene. That different parts of the body will smell if not washed daily or every other day. You could also do a lesson on food from other places. The curry in foods from India tastes wonderful but it can cling to their clothes and make the sweat smell different -- not bad but different. Same as many other ethnic foods Chinese food uses different spices and will change body chemistry.
My daughter works at an IHOP and says she notices on some days she just smells like pancakes.
Hopefully by introducing the kids at a young age to different ideas they will learn to be more understanding of dofferent people and cultures.